I imagine it goes a bit like this... @~@;;

Slipspace is like the tacky "crunching space in on itself to make it smaller then it may seem, finding the space in between particles to stuff a ship through the acceleration caused by protons in their natural orbit. The energy of the protons scraping against the shield in such a way serves as to power a generator which has the sole purpose of stuffing ourselves through the fold of atoms in a CRRRAAAAAZZZZYYYY metaphyiical-psuedoscientific McGuffin which makes sense of this. This wackey affront to modern science creates a 'field' emitted by the ship's gererator as to create a field around themselves as they do what I just said.
With a bigger ship, the space would be bigger, maybe even so big that thethe mass of many small ships could be condensed into an area within the bubble; the stress on the ferrying ship being midigated by the 'Infamous Alto's shield, for example.

A big ship with a bigger engine can travel faster than a smaller ship with a smaller engine. Its like how driving behind a semi-truck and following its slipstream during long-distance travel is pretty good for gas mileage.

Science-fiction.
GM, PLEASE??

((I don't mean to come off the way I do, I just...don't understand the lack of understanding for the classic [URL=http://c2.com/cgi/wiki?McGuffin]"Science-Fictional McGuffin"[/URL] ))

It doesn't need to make sense beyond what the gm says are the conditions for FTL travel in this instance