When people use the word "light" in regards to money, it's most always in a derogatory sense.
You might have heard people say "sorry, I can't afford that honey I'm a little light" or "damn, I would go... but my wallet's a bit light". I understand that light can work in some situations like light beer (less calories) or light cigarettes (less tar) but I don't think people want "less money". imo
With this arguement the best could would be heavycoin or fatcoin, but you can probably see the issue. You can say just about any word will have a derogatory or negative sense. Its just a matter of context. But every biased opinion will see it their own way, yours included.