It's sort of ironic that at the time, there were many complaints that Apple made its devices thin at the expense of more important features. Now that M series MacBooks are thicker again, there are complaints that they are too thick.
I owned an i9 MBP with a discrete GPU. It absolutely was too thin. The CPU and GPU ran hot, it throttled like crazy. It would drain battery while USB-C docked while idling. Worst laptop I've ever owned.
The M1 Max I replaced it with was the opposite. I don't think I heard the fans for the first month. But it was much larger.
Based on the fanless Air, I strongly suspect an M1 Max in the old chassis would have been totally fine for non synthetic workloads and an M1 Pro would probably have been fine in all scenarios.
But I think they over corrected on the chassis design when they were shipping borderline faulty products and haven't walked it back yet.
I speculate they gave themselves a lot of thermal engineering margin to bump up TDP with the M-series MBP design (or perhaps they underestimated how good the M-series chips were going to be) The battery being at the TSA limit of 100Wh is quite nice as well. Another benefit is that it now differentiates the "Pro" line from the rest of the laptop lineup quite significantly. For most people the Air has enough power now and its plenty thin and light. The pro line is for "true" pros with actually intense workflows.
I'm a dev and the MBP line is definitely overkill for me. The 15" MBA handles everything I can throw at it.
By dimensions, assuming the 2015 ("eleven year old") version, the 13" M4 MBA is 0.17" wider, 0.9" deeper, and 0.32 lbs heavier. Where it's harder to compare is thickness. The M4 is 0.44" thick where the Intel one was tapered (0.11"-0.68").
Kind of hard to see that as "HUGE" in comparison. Bigger? Yes, but not really huge.