just make sure they have food out at all times,make sure you show them tons of affection and they will be what they are supposed to be carl is a fat 20 lbs although he is a ragamuffin and supposed to be like that,hank is thin but a is the tallest cat i ever seen and at a 21 lbs he is heavier then a chubby little carl,ms polly is no more 7 lbs you never see her eating lol she is my little girl and is my little cheerleader type of kitty if you know what i mean.if they remain skinny take to the white coats and if everything checks out then they are just skinny kitties,leave the food out... oh change the type of food too,net every cat food is liked by every cat,once i bought the cheapo purina no one would eat it matter of fact Walker the head of the kittys took the bowl it was in(big silver one)and flipped it and back kicked it across the floor i can home 1 day to cat food all over the kitchen floor and to top it off mr pisser pissed in front of the litter box after i seen the mess in the kitchen and didnt clean it up right away because i neded to take a shower.SO it could be the cat food too.