Wow! First week on the site and I am already horrified. But getting back to the ORIGINAL question. My chapter doesn't necessarily have the "fat" reputation, but we have the "reject" reputation, which at a very superficial school means that we do let in the "fatties". A lot of people have said you should wear your letters. As the reject sorority I see that a lot of girls in my house don't often wear our letters, which isn't helping anyone, because then we turn into the reject house that doesn't have a campus presence, when all of the other houses sport their letters and cute shirts everyday. My advice is to appoint at least one day a week where everyone is required to wear their letters/a cute shirt.
Get a girl in your sorority to be shirt/wardrobe chair. The sororities at my school love to sport really cute bro tanks and when we got a few cute ones for recruitment, the girls went crazy over them. For recruitment week we are required to buy the shirts our wardrobe chair designs, every sorority on our campus does this, and we have a specific shirt we designate for each day. Then throughout the year the wardrobe chair designs other shirts the girls can buy, but even if they don't, they at least have a few options from recruitment. We are also required to get a bag, we finally got really cute canvas ones with our letters and symbol this year which are definitely on par with the other houses. Let other girls be involved in the design process and you will see more girls wanting to wear the shirts. Also, we have family letters, bigs are required to get their littles their first pair of letters (sweatshirt, crew neck sweatshirt, zipup, whatever they want), and usually every family member (grandbig, big, little, aunts, cousins

) have the same letters. It is a great way to create a special bond between the family members also. Side note, DON'T get american apparel shirts. They are too tight and are not flattering for everyone. Also, don't think we spent an arm and a leg on all of our recruitment shirts. Check around online for the companies that offer the best deals, we got four shirts for recruitment and the bag and each girl only paid $80! Lots of greek oriented clothing companies offer huge discounts for bulk orders and will also send sample shirts for girls to try on for sizing.
It is important to note that while you should encourage girls to wear the shirts and letters, make sure that they know there are certain expectations for wearing your letters. Girls should have their hair straightened or curly (whichever they normally do), but not a rolled out of bed static messy (I had 8 am and 8:30 three days a week and I never used this as an excuse to not look presentable). Girls should wear a bit of makeup (mascara, a bit of foundation or tinted moisturizer, and some blush). Jeans or pants that fit!
As recruitment chair for the new year, I plan on making these requirements better known to the members. We have women of all shapes and sizes and there are a few of the "bigger" girls that always look extremely presentable and fashionable. Being bigger than the size 0 girl walking around campus doesn't mean looking frumpy. It just means you need to be more conscious of how to dress for your body type. I'm sure there are a few girls in your sorority that are always fashionable and fit this description of being able to dress for their body type. Point them out to your other members, it is a great boost of confidence for these members and it shows the other girls what you expect from them and provides them with an example. It might also be beneficial to have the girls who dress for their bodies give some advice of what to look for when shopping.
Sorry this was a really long answer, but hopefully it makes up for the four pages of arguing that provided you with absolutely no help.
Good luck!