One of the side effects of measuring gold coins in terms of standardized sacks and bags is that it could be used by players for a rough estimate of how much treasure they've found. In fact, probably a lot of market transactions are going to be a matter of tossing a pouch or bag towards a merchant, who will give a quick judge of the bag's heft (and take a peek inside, to make sure the contents are what the buyer claims...)
But that begs the question: what does a full bag or sack look like?
I'm basing the size of bags and sacks on the size of a 5-pound bag of sugar and a 30-pound bag of potatoes, because these are common in grocery stores and just happen to have the same weights listed for bags and sacks in Men & Magic. A sugar bag is about as wide and broad as your hand and maybe two hands high; a potato sack is about as big as your torso. These are, of course, low-density contents, so the bags are at their maximum volume capacity; other contents will take up less room, even when the weight limit is reached.
Gold is about 30 times denser than sugar, so a 5-pound bag of gold isn't going to look full at all, compared to a bag of sugar. It's going to have two or three coin layers in a mostly-empty bag that can't really store much more weight. Adventurers are bound to try, however, so figure that a bag bursts on a 7+ (1d6), and every 10 pounds (100 coins) adds a +1 to this roll; the bag tears automatically at 60 pounds (600 coins.) That's about half full.
The density of potatoes is about the same as white granulated sugar, I'm shocked to learn, but that makes it easy: 500 coins/50 pounds adds a +1 to the chance of tearing a hole in the sack, with the sack definitely bursting at 3,500 gp. Gold's just too heavy.
I picked the volume of a chest based on the volume of a sack, but you could say it's twice as sturdy, or +1 to the roll for every 1000 coins. It will break if filled to the brim and lifted -- but that's 600 pounds, not including the weight of the chest itself. You will probably have a different problem to deal with. A trunk wouldn't even feel any strain at 1,000 coins; it will break at 12,000 coins. A coffer only has a 1 in 6 chance of breaking at 3,000 coins, assuming it's wood; stone or metal would fair better, but add a lot of weight.
An actual heap of gold? That's probably about 200,000 coins. Have fun hauling it out!