I think on small maps the horde build quicker, but they also have a better chance of running into some opposition. Maybe not another horde, but a defense with significant numbers to make it a fight.
On a large map the horde doesn't build as fast, but when it does it just keeps hopping around enough that a defense can't build against it, just too much room for it to maneuver so it never builds to a fight.
So your either in a horde, or you fight a horde. At least with the small maps things are congested/confined enough the you have help fighting the horde.
Fugi,
Yeah for most large maps what you said is most likely valid, however with Fester, the close proximity of fields does usually present a significant defense. The only time that there is none, it is because there are a number of "descent" fights elsewhere. This is a point that I have tirelessly been trying to make for some time and it finally seems that Fester has waved his magic wand

and turned the habitual base defenders

into base capturers,

or even better, satisfied HTC cartoon pilots.

It is pure and simple. If most of the enemies resources are tied up on one task, there must be an opening somewhere in their ranks vulnerable to capture.