But the costumes may as well be on a dressform for how they look on me.

The majority of cosplayers don't physically resemble the characters they're dressed as. I think one of the most important things that makes some cosplayers look great is really counterintuitive - a garment shouldn't necessarily fit you the same way it fits the character, or the way you'd want a similar garmet to fit for your non-cosplay wardrobe. Instead, some costumes can be constructed in ways that will create the illusion of your build and physique being more like the character's. I'm having a hard time thinking of specific example, aside from one small, slim girl who does something really subtle with shoulder pads to affect the broad, triangular torso of a male fighting game character. Depending on how a costume fits, I think there's a lot of potential to scale the proportions of the character design to match one's own size.

Of course, good posing also helps, especially when you can try to match a particular image of the character that's particularly freqently used.