There's plenty of space at the beginning but what about late game when it's displaying 4 digit gold, 3 digit mana, 3 digit metal, crystals, wargs, horses, at least one of each shard and diplomatic capital? I've had mine overflow and I play at a fairly high resolution too.
Knowing the absolute numbers is on the whole more valuable than the rate of income. Permanently cluttering up the display with more numbers wouldn't be particularly helpful. On the other hand, there's no straightforward way to get a sense of your income levels for all your resources. Hovering over each icon in turn is terribly awkward.
My suggestion: make holding down the Shift key switch the display to the delta (e.g. +20 Gold | +1 Horse | +10 mana) until you release it.