Why might the size of the roman empire be a problem for roman administrators?

Bureaucracy! That is the main problem of the empire and mostly the republic. The republic fell because the size of the empire was to big, the events were moving rapidly and there was no time for bureaucracy, voting, debating and so on. When Rome became an empire it didn't of course change immediately

The  ancient egyptians  who  built the pyramids  may have been  able  to move massive stone blocks across the desert by wetting the sand in front of a contraption  built  to pull the heavy objects

Absolute !