I swapped the 3.3g on my '06 for the 4.5g.
It was an easy bolt on.
I had to modify my OEM seat cos the 4.5 is longer, but you may be OK what with that gap already there.
Here are a few DIY tips re the swap procedure (some of the technical details may be different on your '11; read your Service Manual)
a) Empty the peanut before you remove it!
Many XL owners have had a tank with gas sloshing around in it tumble out of their grasp and hit the shop floor like a hard-boiled egg
b) Mind the hoses attached to the underside of the tank:
> the fuel line emerging from the petcock
> the vent (overflow) hose
> the nasty vapors hose
c) You'll prolly move your old petcock to the new tank. Take advantage of this opportunity to clean the filter before re-installing petcock.
d) Your new tank will ride lower than your peanut, so accessing wiring and hoses underneath the tank will be more difficult.
e) Take your time installing the 4.5. Those 'ears' holding the front in place can be a bit of a hassle. Patience. You may have to try several times before it goes on correctly.
f) Remember to reconnect all those hoses! Count them to be sure. If you overlook the vapor hose your engine will run crappy.
g) CHECK FOR LEAKS! Any hint of gas escaping? You don't want that.
When you restart, listen for air leaks. Remember to open the petcock valve. Oh, and remember to put some gas in the tank. Don't laugh--people have done this, then start thinking their ignition needs work.
h) What plans do you have for that tiny old tank?