What foreknowledge? I personally expected Republic to be more heavily populated, myself.
When the whole "Early Guild reservation" system was going on, there were nearly 30% more Imperial guilds than Republic ones, and the Imperial guilds were about 20% larger on average.
Now I know that is only a small subsection of the population, but those numbers gave insight into what actually occurred when the game went live: Empire had a noticeable numbers advantage over the Republic.
Now the problem is that as new people come into the game they tend to join the Empire faction if for no other reason than they know it is where the action is. This is especially true for PvP servers since nobody likes being on the losing end of the numbers game when your PvP flag is constantly on.
Exacerbating the situation is the fact that high-level Republic players are switching to Empire so that they can actually find groups to run Flashpoints and Operations with. They are also sick of being rolled in PvP by better-geared Empire players who constantly have control of Ilum and constantly get WZ pops (even if it's mostly Huttball) and therefore have better gear.
I also personally know of some Republic players on my server who shifted to Empire simple because they were sick of playing the less-polished version of their class. Empire classes have abilities with animations that are not only functionally better but also don't have as many glitches or outright disadvantages as the Republic versions.
This is a problem that is only going to get worse as time goes on.