There are ways to tighten up a loose suit. The effect can vary depending on the areas that need to be tightened up. Seams can be sewn tighter, sections can be cut and sewn. One of my favorites is "Cuffing." Elastic thread can be carefully threaded through the backing, making large areas tighter. This will also allow stretching for movement, or if the weight is regained.
Another suggestion is waiting a bit, and starting a weight loss, exercise program. Suiting can be exhausting if one is in shape, if you are out of shape, it can be rather miserable. If you want to go dancing, or bounce around at cons, meets, parades, you will want to be able to do these things with out collapsing! Depending on the builder, there may be quite a bit of time waiting for an opening. If you go with a big $$$$, high end suit, you might also need to save up cash, make payments before the suit is built. Use this time to get in better shape, wait as long as possible before making your DTD. The end result will be better, you will be happier with your suit, and you can run around and not drop dead after an hour!
Building your own, you can buy the fur needed, then start with the parts that will not change much as you loose weight, get in shape. The paws won't change much, neither will your head. You can then have fun with a partial if you want, while working on yourself, and your suit body. When you are building the body, think about areas where you might need to tighten things up. Be careful where you place any striping, body designs. That way you can build in areas ready to take up when you need to.
