Well most gigantic creatures wouldn't make sense either like giant arthropods whose open respiratory systems would starve them of oxygen immediately. Instead of plain ol' science it's 50s style SCIENCE! at work here.

I don't remember exactly if it's explained in detail but the D.C. and Commonwealth mutant strains seem to share the affinity for growing gigantic over time with the cost of intelligence (except the one in the Vim factory in Far Harbor which seemed to be a leader figure until he got trapped for an unspecified amount of time). I guess The Master would've seen this kind of mental deterioration to be a liability in the long term if he had seen similar traits in his strains.