If I understand you correctly; the starting armies are only useful for 'rush' tactics. If you fail to rush (new player, slow to move - whatever) then the game quickly reverts to standard rts with the starting armies adding almost nothing (i might be mistaken but even at 1:3 it sounds like a very small # for a long running battle). I.e, it sounds like if you can't complete the rts battle in a short period then the starting armies are no longer relevant and it doesn't matter if the initial armies are 1 vs 1 or 1 vs 10 (esp if the person with a single army has a lot of upgrades they can apply to the armies they build on the battle map).
Something like that, yes.
If you have a substantially stronger starting army, you can rush the enemy and the fight will be over fast. Otherwise you will have to do the complete RTS procedure, consisting of base-building, unit production and the actual combat.
In that case it is pretty normal to produce a multitude of the units you brought with you.
I too think that this is a bit strange, but Larian probably has chosen to do it like that to give skilled players the possibility to turn unfavourable battles.
However, the discrepancy is larger in multiplayer campaign mode when not playing in teams, since most maps are made for 2vs2 - 1vs1 means that you have much more resources available, meaning that you can produce more and faster.
Also, as Raze has mentioned, you can adjust the build-cost multiplier in the advanced options.
I recommend to try it out, it makes for a different game, since with 2-2.5x recruit costs the battles depend much more on initial units as you will be able to produce much less.
