My hamster is too skinny. . .?

I've taken my little girl to to two different vets due to two different problems. Both have made absent-minded comments about her weight, saying she's a bit thin. She is easily able to use the standard crittertrail tubes. She can even fold in half and turn around in them (Don't worry. She hardly does this.) I think it's just because her cage is big, so she has a lot of climbing and waddling to do to get anywhere. And she loves her wheel. She runs about 3 or 4 hours straight. I know most people have the opposite problem, but what can I do? I already leave a constant supply of food (I use Ecotrition organic) and give her 2-4 strawberry yogurt drops daily.