I don't know the exact answer, but I can think of a few reasons why
the AE86 sump is the size/shape that it is. To keep the hood line low the engine has to
sit as low as possible. Only this means the sump gets closer to the road, and it has to be
tucked up high enough to be out of the way of accidental scraping damage.
On the AE86 the bottom of the sump is about 1/4" higher than the front crossmember.
The position of the front cross member also dictates that the sump be a "front bowl"
design, as that is the only place where a bit of depth is possible. The rear part of the
sump has to be shallow as it sits on top of the cross member. So packaging issues.
The 4AGE is a water cooled engine, and as such, the engine cooling system will take
care of keeping the oil in a stock engine
at an acceptable temperature. Additional oil
cooling only becomes necessary when the engine is modified to produce a lot more power
(and heat) than stock. Or the engine is subject to continuous operation at high loads
(towing) or high temperatures (Death Valley).
As far as the quantity of oil goes, there has to be enough to maintain full flow thru the
engine at all times. This means taking into account how much is in the galleries and head
while still keeping the pickup submerged. In this respect a small, deep sump is good, and
also helps prevent the oil moving away from the pickup under lateral acceleration. The
stock AE86 sump seems to be fine for stock and mildly modified engines.
Race and rally engines that are liable to sustain high rpm and high lateral acceleration
on bends need to take special precautions. They need to ensure that there is adequate
cooling and sufficient oil quantity to keep the pickup submerged under high lateral G's.
Winged and gated sumps (or dry sump) and an oil cooler are more or less standard
modifications for these engines.