During TOS, it was implied that the Constitution was the most powerful ship the Federation had.
No it wasn't. In fact, it's not even implied to be the most advanced in the fleet.
More to the point, even if it WAS the most powerful -- which TOS gives no indication whatsoever that this was even remotely the case -- that still doesn't imply "biggest."
Wait, you just turned a good premise into an unwarranted conclusion. The conclusion is: the Enterprise is the most modern starship available to the Federation, and probably the biggest...
"Is probably the biggest" doesn't follow from that conclusion, since modernity and largeness are mutually exclusive.
FYI, the most modern
warship in the U.S. Navy right now is the Littoral Combat Ship, yet for entirely different reasons it is also by far the smallest.
My overall point is that Enterprise appears to be slightly above average in comparison to a series of much older vessels with a similar hull configuration; reasoning that other ships of her generation would be similarly enlarged, it stands to reason she is actually pretty average for her generation, or at least, for her particular design system.