Oops! I reread the description, and you're right. It should probably weigh the same as a normal rat.
The skills look good. Are you also going to give 'em the normal rats skills bonuses? Here they are for quick reference...
Skills: Rats have a +4 racial bonus on Hide and Move Silently checks, and a +8 racial bonus on Balance, Climb, and Swim checks. A rat can always choose to take 10 on Climb checks, even if rushed or threatened. A rat uses its Dexterity modifier instead of its Strength modifier for Climb and Swim checks. A rat has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. It can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line.