According to a recent TV show, most "designer mixes" are produced with the female being the larger breed. Some dogs, like pugs, can need c-sections because of the large heads and narrow hip leading to problems giving birth, so they use the larger breed for the mother.

I think Mikey could absolutely be "puggle" - half beagle and half pug, it's just that his genes swung more to the beagle side than pug, and that sometimes happens with mutts - a.k.a. designer dogs! After all, think of human siblings - we all look somewhat different, even with the same two parents ... same rules can apply!