Hi there. Day and night sleep are controlled by different parts of the brain--for night, melatonin and his natural circadian rhythm give him a much stronger drive to sleep. During the daytime, babies' drive to sleep is not as strong and many times they just don't feel like going back to sleep after a short nap--they've already had a bit of sleep and in their minds, they're ready to get up and play!
For us, overtiredness never really results in short naps--it's generally undertiredness. They are almost always caused by a too-short A time (and occasionally factors like teething or crawling). When dd has a too-short nap, I know that her routine needs tweaking.
HTH.