I think OSRS did restructure to do something from Rsgoldfast's blog

Inferno in June, and Fossil Island in Sept; not to say those were the only updates, but the majority of OSRS gold what was in between was smaller stuff or moderate at best.

So given the way the pacing did change across the time RS3 transferred to Expansions, I think OSRS did restructure to do something similar going from a lot of moderate to big updates to massive updates.

And while they did scatter small and moderate updates around the huge updates, there tended to be a shortage of significant updates after them in comparison to the prior release cadence.

Therefore it wasn't due to a lack of developers since they produced more updates with fewer Devs.

It was the shift to making bigger releases rather than dispersing them . For instance, Kebos might have been split with Farming Guild, Brimstone/Konar, Aerial Fishing, the Quests, and also the Cheap Runescape gold Diaries all being independent updates over months rather than one massive update, but they chose to pack it all together even if that meant their were not many devs left to move things around it.