To be fair, you are only taking one dimension (overall height, I'm assuming) into consideration, and as your example of a mechanical you use a device with a kick which makes it longer, and kind of makes it a regulated vs regulated comparison for the vast majority of people.
A more fair comparison would be a Nemesis in 18500 mode with no kick to pretty much any APV/regulated mod out there, and the Nemesis is smaller in every single dimension, except maybe thickness on some of the smaller/lower power APVs. Additionally that Nemesis is capable of over 50 watts, regardless of the battery's charge level, and staying within an AW IMR's safety rating. Compared to 50W box mods, the 18500 nemesis is definitely smaller in every single dimension, and still more powerful.
I'm not trying to say APVs are bad because of this, but in a fair comparison they lack power and/or are larger when compared to mechs. Whether those things are important is the question, because they are definitely true.