Yes, there are plenty of ways to make a person look slimmer in photos. Much of it will have to do with the photographer....so things like the lens (focal length) which relates directly to how far the subject is from the camera. The farther away you are, the flatter the image tends to look. That doesn't necessarily mean thinner, just flatter. But you typically want to avoid being too close, as that will lead to wide angle distortion...but then again, that can be used for great shots, especially when the photographer knows what they're doing. For example, a wide angle lens that is above your head, will make your head look large and your body look much smaller in comparison.
Another thing will be lighting. A general rule is that 'short' lighting will make people look slimmer than 'broad' lighting. This has to do with which direction the model is facing, in relation to the camera and the position of the key light.
As far as posing, keep in mind that the more 'square' you are to the camera, the larger you will tend appear in the photo. That's why we usually pose people with their body/shoulders turned to the side (either a little or a lot). Of course, some body types won't look as good when seen in profile, so they may look better from a more frontal angle. Of course, the intent of the photo should play a part in the posing. If you are dressed up as an imposing, strong character, you may want to pose straight on to the camera to emphasize those characteristics.
There are some little tips, that you may have read already. For example, if you put one leg closer to the camera, keep your weight on the back leg. A very useful tip is to shoot people from above. This can make their body look smaller (in relation to their head, as I mentioned before) but this also has the effect of making the subject look up to the camera. As a person looks up, they stretch out their neck, which will slim it. And as you're a girl and this is cosplay, I'd guess that you might be showing some cleavage and by shooting down, they can really emphasize this to your advantage.
On a similar note, one thing that helps most people to look a touch slimmer, is to push their jaw forward while posing. This isn't usually a natural feeling for most people...actually, many people suck their chin in when they see a camera. I have to constantly remind and reinforce people to push their jaw forward. It stretches the chin & neck, making them look slimmer. Practice this in the mirror and get used to the feeling. Then try to do this during the photo shoot. It's hard work and even harder to remember to keep it up, shot after shot...but it really does work.
Lastly, don't worry too much about being a "rather large girl". Being comfortable and confident will trump that and can really show through in the photo. I'd guess that with cosplay, it will help if you really get into character and forget about any shyness or awkwardness that you may normally feel. Have fun with it!
