Fat David is probably well into the realm of parody and would probably have no issues in a commercial application. Which brings up an even murkier subject with copyright Fair Use.

Anyone is free to make a statue and call it David, as the name David is referenced from the bible and is not the property of the current statue owner. In fact there are numerous master work statues named David that I know of such as Donatello's rather effeminate version, or Verrochio's David. So the name itself isn't protected by copyright and you could quite easily make a human figure study called David 2 without any real issues, as long as it doesn't reflect a likeness to the protected Michelangelo version. The likeness is what will get you into trouble, even if you call it Peter or Mary.

Where legal issues will definitely occur is firstly when mechanical copyright infringement occurs i.e taking a picture of David and using it to promote a tour company in Florence without correctly licencing the use of the image from the statues owner. Or manually, if I were to employ a sculpture to create 1/20 scale clay copy, of which I could create a mold for the creation of unlicensed knock off statues and sell them to the local street vendors. So the main thing here is generally all copyright cases are based around competitive commercial application of owned IP. Anyone is free to take personal images of the statue for their own use with certain conditions. But to take a photo of the statue and then claim copyright over the image and place the image in the public domain for anyone to use is likely see you in court with the actual copyright holders of the statue. I would have to say most cases of copyright with David have been pretty cut and dry. If you respect that the statues owner has the right to licence commercial representations of that IP and that anyone using its likeness without permission is in breach of that right. If they attempted to claim that your reclining ZBrush model was derivative of the David Statue because the way its little finger was posed similarly, it would be a very long bow to draw and not really within the interests of the statures owner in pursuing as there is no direct commercial competition even if that little finger was inspired from the statue.

What the copyright holders actually does own in relation to David is the likeness of the statue, which is kind of an abstract idea tbh as likeness is very subjective. But if you take a look at the three statues of David I mentioned earlier, they are all the same subject yet all are very different from each other. Easily recognisable as their own IP with their own form and style. You can be inspired by anything, but generally most people can see where the point of inspiration ends and direct copying begins. Though the line occasionally does become blurred and copyright at these times tends to fall on the time established of the two similar works or to the party with the deepest pockets.

You tend to find that very few small copyright incidences will ever make their way to court or through any serious litigation. The first thing you will see if you do step on the toes of any major copyright holders, not to mention any names like Disney or anything, would be a cease and desist letters. Most people at this point will decide it just isn't worth pursuing and drop it. But if you do decide that your little black eared, red panted anthropomorphic mouse character is in no way a copy of Disney's IP you better have some very deep pockets and some strong documentation showing how your character was developed, even inspiration from Mickey in cases like that would be an instantalose for youself. Cases like that are more about making an example and setting precedents, so regardless of how much money you have the fines and penalties in a likely loss could be astronomical. Buildings are another issue and will likely come down to how aggressively the copyright owner wants to protect their assets or whether licensing is required. The Eiffel Tower is an interesting case where Photographs of any kind, even personal use, are illegal at night because of the copyright protections of the light displays.

At the end of the day, a background working knowledge of Copyright is about all that is possible or needed, as it is a rather mercurial subject likely to change shape according to specific scenarios . Often copyright will be breached and never pursued by the legal entitled entity as they have no interest in it or it may even be helping them promote their IP. Other time the copyright holder may aggressively attack the slightest infringement to the point of dragging kids at local fares off of blue trains with smiley faces and demanding it be repainted and the face removed or destroyed completely.

I found it is best to always er on the side of caution or if you are unsure do your best to communicate with the copyright owner and ask what requirements they may have for licensing their IP, sometimes it is cheap as a cup of coffee other times they just like to be informed of how you found the item useful. Always try to get permissions in writing as it will help you if any issues come up later. The license you attach can also have a big impact on how aggressively perceived infringements may be pursued. But generally, be inspired by everything, do not copy anything protected directly whether mechanically or manually, make sure you follow any rules if your usage borders on derivative and you won't run into any real issues. And if you do act accordingly with what you know to be based in law. Don't challenge someones Copyright or IP on a hunch or personal intuition and always seek professional legal help if in doubt. And you will pretty much have no issue with copyright law.