Going on what CosplayGabi said, if you alter it right, it really won't be that noticable to all but the most anal to details.
For example, a Sunseenli did Tifa. Since Sunseenli is full figured (and totally hot I might add), she used a full leotard for it and made the skirt slightly longer in her costume. I didn't notice she'd made any alterations on that part until I was thinking about how you could make Tifa work for a larger woman.
I'm full figured myself, and try to look for styles of costumest that flatter my body type. That means that I may end up doing some pretty obscure outfits of a character, but that makes it more fun for me! (I enjoy people being confused and the occasional, "HAY THAT'S NOT WHAT THAT CHARACTER WEARS!" Just means I'm spreading interest to find out exactly what part of the series my outift came from! <3)
Things like school fuku with a slightly longer skirt and gowns work great! (Fuku are also great for beginner cosplayers!) The bows that most girls wear in anime take away from the bust area and pleated skirts look great on everyone!
What also looks great are dresses that flare out a few inches below the bust line. This makes the thighs/ass look slightly thinner and the flare line takes attention away from the bust.
Colour and fabric choice is also something to be aware of. Matte fabrics and darker colours are flattering on larger bodies. (There's a reason they say everyone should have a little black dress!) Also, sticking to colours that go with her skin tone will work, too.
"Wanted: Somebody to go back in time with me. This is not a joke. You'll get paid after we get back. Must bring your own weapons. Safety not guaranteed. I have only done this once before."