When you say 2.5 POUNDS do you mean 2 pounds 5 ounces. I have a 2 year old that weighs 2 pounds 11 ounces. I noticed when we got a new puppy (not a yorkie) a little while back, Candee would eat the puppy food (she was not suppose to, but she did sometimes) she gained at least 5 ounces it's not alot but in these small breeds it makes a difference. And also after I spayed her she put on a few more ounces, I was happy

, thats how she got to 2.11ounces..
Oh, yes maybe substitute a little boiled chicken breast for a meal might help among other treats also..
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Candee, Bear, Daizy (in spirit, R.I.P) and Pepsi's mommy