You need to have a high DOP for the larger planes. 250k min. for 757. 275k for 762E (7600nm ver). I would say the rule of thumb on the bigger planes is 300k dop. Thats just to break even. Remember that maint. increases every month. In 3 years(game wise) a 762E cost just under 6M in maint. So @ 300k DOP, after 3 yrs, your making 1.2M in profit off that 762E. (300*24= 7.2M-6M=1.2M) <Gate cost not included>
Also, you have to look at the cost to buy that plane. 150M for 762E. You dont spend 150M to make 1M overall profit. Thats just not good business.
If you able to make a high DOP thats great, but for most of the players, only a few large airplanes would suffice.
Then again, if your bases are ATL, ORD, LAX, LHR the likes. Then it doesnt really matter, you could fly 744s all over the place and prob break even. Bigger airports = Bigger income = Bigger airplanes. ^_^