Well clearly something is different if the iPad Pro can handle quite a bit thrown at it with its severe limitation on RAM. What is different, I do not know. But something is.
It has to do with both the operating system and especially the application optimising for low memory use.
Very often you have several algorithms to solve a problem. Often you can decide between an algorithm with low memory use and high CPU use or vice versa. Under iOS the programmer will more often choose the low memory algorithm.
Also with more memory the application may choose to have larger buffers and keep more data in memory.
Third, your app will be rejected by Apple, run poorly or crash more often if you use a lot of memory under iOS. This creates a strong incentive for the developer to put in a lot of effort to reduce the memory usage of the app.
On the other hand, all those people saying you need a lot of memory on the Mac side tends to forget that MacOS and the applications will use more memory if its available. The mistake they are doing are looking at the usage in a 16Gb system and seeing the OS using all the RAM, thinking therefor they need more RAM.
For most regular users, 8Gb RAM will be enough.