In practice, of course, APS-C sensors have been higher density than full frame sensors (probably something to do with reject rates - bad small pixels on a FF sensor would mean an appreciably more expensive chip to throw away, so they probably use older production processes) so, assuming the lens is up to the task, you can pick out more details as well - generally good for wildlife once again.
Its more difficult to make FF sensors: From Canon White Paper
"the circuit pattern of a fullframe
sensor is too large to be projected on the silicon wafer all at once; it requires
three separate exposures (See page 53). This means that the number of masks and
exposure processes is tripled. For now, appreciate that a full-frame sensor costs not
three or four times, but ten, twenty or more times as much as an APS-C sensor"
Aligning those masks is very difficult, and as photosite size decreases, it must be even more critical.
I wonder if thats why Nikon has kept with 12MP on their own FF sensors. Sony and Canon had the technique to do a finer alignment for a reasonable cost?
APS-C and APS-H do not require the triple masking and alignment process, so they are much easier to make.