There are several reasons each machine uses different amounts of RAM for similar application processes.
Though you don't specify the OSs... I'm gonna assume the older machines are running older macOS systems. Those older system require less RAM to function well. The most current macOS has much more stuff going on even at idle than the older systems.
They may also address RAM differently in ways such as 32-bit v 64-bit chunks. 64-bit being larger chunks of memory at one time.
Your 27" iMac is also driving a much larger monitor than the others, and while the GPU with its onboard memory probably carries the bulk of that, the machine may allocate some resource to that task as well.
Then there's something called "virtual memory" where the system may recognize that the actual RAM available for use may be limited (4-6 GB in the smaller older machines) and therefore will use space on the HDD as virtual memory. This is being used but would not show up in Activity Monitor as RAM usage.
So there are many reasons why one machine may appear to use much less RAM than another, especially when comparing older systems to newer.
:)
