Do you feel certain games just play a lot better on the iPhone than the iPad? If so what games? For me it's the rhythm games.
It's probably that the shape and weight of an iPhone makes rhythm games smoother and easier to play. Your hands don't cover as much space and you don't have to move your fingers as much.
I can tell some games were completely made on an iPhone and made iPad support just as an afterthought.
It's worse when say the game goes in the middle of the screen and your thumb just can't reach it in time.
A couple of the Final Fantasy games I've played thus far... IV and IV: The After Years. For the latter, I tried playing them side-by-side, and while the IpT version has some area around the Ipad's screen cropped out, you can still see most of it. The pro with the IpT is that you can easily reach the whole screen with your fingers. No need to wave your hand around the larger Ipad screen. That said, my IpT5's battery is dying after 3 years of use, so I've gone ahead with the Ipad Air anyways. It does have better hardware too. Ditto with Ascension: Deck Building Game. It's easier to reach across the screen there, esp. since dragging and dropping is typically quicker then double tapping and then tapping on another button. https://itunes.apple.com/us/app/ascension-chronicle-godslayer/id441838733?mt=8
Pretty much anything in portrait mode is better on the iPhone, like all the DragonQuest ports. And Michael Brough games, like 868-HACK, feel oversized on iPads.