With the longer spools the line level doesn't drop very quickly .

If you look at a classic reel like a penn 850 its a short deep spool . When that sort of reel is cast the line level drops very quickly . Big down side of that is the line either gets thrown out in big loops which rub on the rod and or rings excessively adding lots of friction.

If you have a nice limp line on you end up with the line rubbing on the edge of the spool which also adds friction .

The long shallow spools as Mark has stated throw the line in fairly small loops and as the line level doesn't drop that much there is far less friction .

The way the line is laid on the spool also helps with the best casting reels having a super slow oscillation so the line is stacked on the spool very very closely.

Freespools had a few distinct advantages where the line is not been thrown in loops so there is less friction and with the spool actively throwning the line off the spool due to its momentum so less friction again.

Getting the braking of freespools right is the big trick . Too little braking and its birdnest time . Too much and you lose distance ..

My fishing freespools are all set up with lots of braking so they are nearly idiot proof .