Actually your logic is not true. If there was a profound ppi increase, the iPad pro would use maybe a bit more ram but not quite much at all. Both ipad pro's have remained with apple's standard 264 ppi. The iPad pro 12.9 uses its 12 core gpu to display the over 5000000 pixels and it's enhanced resolution, it barely bothers the ram at all. The iPad pro 9.7 uses a 10 core gpu for its resolution. Apple perfectly compensated for the iPad pro 12.9 resolution by having more active gpu cores and keeping the ppi at 264 ppi which should not bother the ram much. Only applications and software affect the ram profoundly. Double pixels does not mean double ram usage......
Based on the way iOS handles putting apps to "rest", I am not a believer that more RAM will increase performance "profoundly", unless we are talking small amounts such as 512 Mb where the device is starved.
Obviously there is no direct correlation between pixel count and RAM requirements, but clearly the massive jump in pixels from the 6S+/iPad 9.7 to the iPad Pro 12.9 justified a need for additional RAM. The GPU in an iPhone doesn't have it's own allocated RAM as a Mac or PC (usually) does, where else do you think it comes from?