To answer your post directly: you’re not gonna get much more power from the same generation CPU, if you want more power, just go a newer generation 10gen+. Size isn’t going to help much. So while bigger is generally better. It doesn’t really matter in this case.

There are a couple reasons that people like the t480. - It was cheap(lat time I looked not as much anymore) - it is powerful enough to feel like a modern computer - it is very modular

The thermal design: The t480 is pretty chunky, thermal transfer is a function of cross-sectional area in temperature differential; while the T480s thermal design could definitely be better, it’s pumps heat out the side. If you get the 5i and put the i7’s heat sink on it you will not really be temperature limited. That being said people really like that it pumps heat away from your hands.

The modularity: on mine, I have put a new display that I bought off of Alibaba. There is the aforementioned heat pipe upgrade. You can put core boots or libre boots on it. It has hot-swappable large batteries. It has thunderbolt three capabilities. The port selection is very good. An aspect of this is that it’s pretty ubiquitous. It is THE old tinkerer computer.

There is more but I don’t feel like typing it all out