In the original D&D product line, Gygax decided gold coins weigh 1/10th of a pound. He did this to make it easy for himself to calculate treasure weight for encumbrance rules. Basically, he was being lazy. The story gets weird though. Gygax liked this so much, he decided to measure the weight of everything in the game in coins. This is simply bizarre for a multitude of reasons, but mainly because 0.1lb doesn't match up with any other extant measuring system. It brought extra attention to the unusual coin weight.
The Gygaxian Coin has gotten a lot of criticism because of its weight though, with people poking fun at the thought of dinner plate sized gold coins. Now, I'll agree, a tenth-pound of copper or silver will indeed make coins in the 5 to 6 inch diameter range, no doubt. But gold is damn heavy. Excuse me while I pull out my ultrasonics text book and do some volume calculations based on material. Let's see just how big these would be.
1lb. of gold has a volume of ~25 cubic centimeters. So 1/10th of that has a volume of 2.5ccs. That's not much! Ok, let's say a Gygaxian Coin has 2mm thickness, same as a Canadian loonie. We now have 3 of the 4 values needed to calculate volume, we just need diameter. Luckily, the calculation for cylindrical volume is straightforward, and we just need the first value, so we can just reverse the calculation. I'll just plug this into math calcumalator...
At 2mm thickness. A 1/10th pound Gygaxian gold coin is 4cm across. About 2 Canadian loonies across, or about an inch and a half, around the size of a poker chip. That means the 1/50th pound coins in 5e are actually thin little wafers of gold, barely even coins, and probably quite easily damaged.
It also means you could literally spray paint poker chips gold and have to-scale accurate coin props.
As for coppers and silvers, they have about half the density of gold, so they'd have to be twice the volume. The easiest way to do that is to double the thickness to 4mm. That is one big, heavy, crude, primitive type of coin. It'd be more like a miniature precious metal bar than a coin. Platinum on the other hand is actually more dense than gold, but not by much, so those coins would be about the same size as the gold pieces.
I made one. Sans minting, this is what a Gygaxian tenth-pound gold piece would look like.