One of our rescued dogs is a small 8 pound Maltese mix. Bella came into our home last year being fed the same brand (Natural Balance Duck & Potato) that I've always fed my 20 pound Lhasa but in the "small bites". I successfully transitioned her to the regular sized kibble, mostly because of the availability (lack thereof) of the small bites. She does just fine with the size of the food, I mixed what the rescue group gave me with what we had and she transitioned.
I'm wondering if you could mix the sizes together for a little while and see how your dogs like it or if they have any issues with the larger kibble?
If you do change the brand or formula of their kibble I would suggest making it a slow transition, mixing the new with the former brand and increasing the amount of the new brand over a number of days. Much less chance for stomach upset that way.
Good luck...
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