You can't really compare it like that because it doesn't scale linearly. Every update is different because of things like more renders added, different chapters reworked (and different renders), maybe Summer had an issue which caused the development time to skyrocket (think he became sick a couple times, and the recent dog sitting trip also comes to mind), so now its not a proper indicator of time taken to produce the update/ renders...
Then, like I said before, he really improved on his work ethic which caused the production time to sharply decrease, as seen in going from 4 Full to 4.5 beta. Then some issue came up which made the 4.5 Full release so lengthy... and so on and so forth.
When you add all those variables into the length of time needed per update, there's no reliable model which could predict development time for future updates with any amount of accuracy. So to say that 25% more renders = 25% more time is a bit... I dunno. Facile.