I think our capacity has expanded also. Not nearly quickly enough, but we can learn more now than we ever did before (or at the very least, we can cut out a lot of the noise in our learning experience).
Also, at least in math, while there is certainly more content than one can learn in a lifetime, with more content being created every day at an increasing rate, many different abstractions have been developed and clever analogies applied in order to simplify new learning.