I don't believe that it's 'necessary' to have a higher wpg over a 10 than a larger tank.
It is easier to manage the wpg ratio in a smaller tank than in a larger one. In other words 50 watts or 5 wpg over a 10 is much more manageable than 5 wpg, or 275 watts would be over a 55.
I currently have 28 watts over my 5.5 gal. or about 5 PG, and 56 watts over my 10 gal. with few management issues. Both tanks, BTW are CO2 injected. It would not be as easy a task without the gas.
On the other side of the coin, I've had 12 watts over the same 5.5 and 15 watts over the 10 and was able to grow nice plants, more of the lower light variety in those environments. I do remember growing some nice red Tiger Lotus in the ten at 1.5 wpg.
In a nutshell, I'd say that you have more room for a variety of systems in a smaller tank, IME.
Len