Well, the fruit and the pet store treats together with the pellets might do the weight gain thing.
Are the store treats alfalfa based? Most are,so we (I) do not give any of the store treats...just a small piece (1/6-1/8th of an apple (depending on the size) for example) them the fruit once or twice a week several days apart.

That's because if mine get fruit every day, they get runny poop.

They are just so darn cute I want to spoilt them and give them treats but I found it backfires ()
!!!