Yeah but what is the average amount of units? I find that once you get 10+ units in the fight then starting units feel like they really matter while if you do the tiny 2 unit vs 2 units fights the AI loves to pick they won't really ever feel very large.
I used to hate the conversion rates, but they have improved a bit since the start of beta and now 10-15 unit (on the strat map) clashes actually feel like you start with something. It might be that you just don't like it because the AI loves picking tiny fights as opposed to big offensives. That's just my opinion though.
Early on I usually have rather small amounts of units I bring into the fight, but it increases as the campaign goes on.
Of course, when you bring large forces, those will matter more, no question.
Lately I started a new 2vs2 campaign, while before I mostly played with 4 independent factions. This again can change things quite a bit. Suddenly you play with 4 players on 2vs2 maps, which means that you end up with less support and ressources and consequently less units compared to a 1vs1 on the same map.
So it might be that I will have less issues with single player, since I suppose that in SP the maps will be balanced around 1vs1 (or whatever the campaign at the point asks for), rather than being made for 4 player matches.
Btw., what do you mean by "the AI loves picking tiny fights"? That the AI tends to invade with small forces? Because on the RTS maps I've seen some quite large offensives (see my GBOD topic). Although they usually happened late in the (RTS-)match when ressources start to run out.