As far as I know there is no way to increase the limit of units a map can hold. Which by the way is 1,650 units, this includes buildings and doodads.
The "Cannot Create More Units" error was built into the game to prevent massive lag time online. Think about it, even if all the players are using an uber-fast Cable connection, if there are way too many units running around, the lag all those units create is nightmarish at best.
Also, as far as I know, nobody has been able to make any map that allows more then the 1650 unit max of the game. Also, ALL of the editors can only allow the max units of 1650, I've checked them all. From normal StarEdit to SCX-tra, to StarForge, to SCMDraft. If you try to place that 1651st unit in them, they will either crash(StarForge did that to me once, LOL), or give you an error saying you cannot place any more units.
I ran into this problem creating several maps. Not only that, but there is also a limit on locations as well, which is 255.
The game will only allow 1650 units max. No way around this as far as I know. Except MAYBE a MOD of some kind, but so far, I have yet to find one of those either.
The reason the smaller maps seem to be able to have higher CCMU limits is because the units are packed in more then a larger map.
Keep it in mind that, it doesn't matter how big or small the map, there are two major limits to what the game will allow, and those are;
1650 units MAX, this includes buildings and doodads(bridges, ramps, trees, etc...)
255 location MAX.
Hope that clears things up for ya.
Good day to ya mate,
Ry