The reason a 9mm is considered underpowered is because of two major reasons. Mass and Size.
The 9mm Parabellum round is an incredibly fast moving round which can cause hydrostatic shock if placed well, but unless you shock vital organs enough, the person you shot isn't going to go down easy. Hydrostatic shock being based on transfer of energy, in which E=mc^2, showing that the transfer of velocity, or c=acceleration, is more important than actual mass.
When people say it's underpowered, they're usually comparing it to the legendary .45 ACP. The .45 ACP is a slow moving, heavy, hard hitting round in comparison. The .45 ACP is barely able to produce the hydrostatic shock, because of the much slower velocity, though the mass is greatly higher. Because of this, the .45 ACP +P, .45 ACP Super, and the .450 Rowland were developed to reach higher velocities with identical dimensions.
The other issue is sheer size. The parabellum is much smaller, so it will penetrate longer before coming to a stop compared to the .45 because of surface area friction. There's much less surface area, which means that it will mathematically have a smaller ratio of real impact vs total force, because impact is the ratio of force over time (F/t).
The .45, being much fatter, and slower will stop quicker, which means a greater ratio of impact to force. Smaller time, greater impact. This is the reason why the .45 is called a "1 shot stop." No matter where you hit the person, they are going down, unlike a 9mm parabellum which may require several shots before they go down.