"Perhaps the 738 (which I am not familiar with) is operating closer to it's maximum performance most of the time?"
I'm qual'd in 737NG/757/767. I would describe the above statement as somewhat inaccurate. When calculating takeoff performance, we look for the lowest thrust selection that can accomadate a given runway length and temp. However, at anytime during the takeoff roll we can TOGA max thrust of 26K. While the paperwork from dispatch often reflects balanced field conditions, we can, at pilot option, increase the takeoff performance margin. So while the 738 may appear to operate near "max performance" it is something we do intentionally for economy reasons. There is often plenty of thrust held in reserve that's a simple TOGA click away.
When I pull the paperwork, and I see a takeoff weight spread between planned takeoff weight and max takeoff weight of less than 5000# (close to balanced feild conditions)....I will often increase my thrust selection from 22K to 24k or 24k to 26k if I don't like the dispatcher's planned weight spread.
The 757/767 in my experience doesn't operate near max performace (balanced field) very often. The is thrust available is too much IMO, and from a wear and tear perspective....wasteful. A recent article in the USA pointed out that 757 is losing popularity to the 738. And Boeing is considering shutting 757 production down as a result. This is because a 738 can do pretty much the same job for less money. The 738 is, in many ways, a much better airplane than the 757. I love flying the 757, but the 738 is a better buy for most operators.