It's 3-1/2 inches taller... which makes it seem much bigger.
Note that Honda recently 'derated' the HP for the GX25, 35 & 50. (They changed units, from BHP to 'Net HP.') I don't know whether the HP difference is real, or whether there's just a difference in KW between a Brake HP and a Net HP....
The key difference between the two engines is this: The GX50 has more torque at a lower RPM (GX50:2.2 FtLb@4500, GX35:1.4 FtLb@5500.) Both engines develop maximum HP at 7000 RPM. This means that the GX50 will pull better when the engine slows down - when going up hills, or you're accelerating.
But, nothing comes for free... 5 more pounds, plus, the engine weight is centered higher on the bike. In addition, while you may be legal in CA, in many states, 48cc's (or more) mean that the bike has to be registered as a motor vehicle.