I would agree with this for AoW 3, but not for Planetfall. The reason for this is that XP for units is largely secondary. The units don't get much, and even if Champions upgrades are implemented (which will happen, when you look at the mod tools) this won't change. XP will give units more HP mainly, slightly better hit chances, and possibly an ability at last. Mods will be more important here.
With Heroes, this will look differently, though, since you can not onlyupgrade them as in AoW3, you also can buy abilities for the points gained that are quite powerful, and stats as well. So basically, yes, you go in with FEW units - but preferably with HEROES. Going in with units doesn't gain much, because the gain isn't so great. (If champions upgrades are implemented, they will gain a regular health upgrade for the full XP cycle, that is, A T1 will need 40 XP to gain 4 HP.)
Bottom line: when you play with FEW heroes (or only the leader), you will try to fight small battles with MANY units to give many units the first levels (making fights easy) and big battles with all heroes you have. When you play with more heroes you will put them all in one stack and do all important battles with them.
Great.