Generally speaking, the NT in a 3-4 scheme will need to be bigger than the DT in a 4-3 scheme. Same is true for the 2 DEs in a 3-4.
The extra strength is needed because the down linemen in a 3-4 are more often called upon to plug 2 gaps. That takes more strength and the bulk doesn't hurt either.
The 3-4 asks the linemen to take on more of a sacrificial role, occupying blockers and plugging lanes to allow the LBs to make the plays and rush the passer. You don't see the linemen in 3-4 defenses racking up huge sack totals.
Look at the Patriots starting DL.
LDE Warren 300 lbs
NT Wilfork 325 lbs
RDE 310 lbs
Here's San Diego:
LDE Olshansky 309 lbs
NT Williams 348 lbs
RDE Castillo 290 lbs
And the mother of all nose tackles, Ted Washington, tips the scales at 375. He's flanked by Ends weighing 320 and 325.