Hiya - I know your LO is much older than mine, but mine has only started taking long naps over the past 2 or 3 weeks. I've noticed INSTANTLY that he is much more tired than he was when he was taking short naps. I do think it is a matter of good sleep encouraging good sleep, I also think that before, he was running on adrenaline and very hyper from it! I think now that he is getting some good naps in he realises a bit more of what he's been missing and he is staying relaxed enough to want to sleep more.
I compare it to the way I suddenly was WAY more tired when he was 10 weeks old and started STTN and when I woke at 7am from an 8 hour sleep, I was way more tired than the nights when I was up 4 or 5 times. Just that my body had a good chunk of sleep and realised that it needed more!
But, glad I'm not alone in this one as I also thought it was a bit odd. We are combatting it by keeping his A time at the same as it was, but more low key. I did try a couple of days to put him down earlier but it only resulted in short naps again, so I ditched it right away!