Okay, from my friend Annie, vet extraordinaire:

It is True! Please let him know this is manufacturer decision. Not the money grabbing vet. Jeez. Actually the mark up isn't that big.
Also some will have a different concentration per dose eg. One might be 5ml/mg and another 7.5.

This is so the pills are the same size Otherwise large dogs might have I take multiple or one the size of a dinner plate. This is VERY true of cat meds and we see many deaths caused by giving cats half a dog dose as (not this product ) they require different dosing. Hope this helps.
So don't just break a larger dose pill in half, be safe for you and your pooch!