One of the reasons that Spelljammer is so small is that they left out detailing most of Spelljammers traditional different Wildspace systems like Realmspace, Krynn Space, Herospace, Clusterspace, etc..., which is what like 75% of the classic setting, instead deciding to turning it into Treasure Planet: The Next Generation, which takes alot less space. It likely they decided to focus on the Mists of Ravenloft instead of the domains.
Given that Planescape will likely double as a manual of the Planes, I suspect that it will be much larger and there is simply far, far, far more to detail. Sadly that likely means more expensive too (although they will likely release the individual books in the slipcase separately for both Spelljammer & Planescape, just like Monsters of the Multiverse). Spelljammer leans hard into relying on other settings like FR for Planets to visit by the sounds of it. It sounds like they concider the wildspace of settings like FR to be part of that setting and not Spelljammer. So that is why it's so skinny compared to what you could expect from Planescape.