iTest process will take memory equivalent of:
heap size + stack space for different threads + code space for iTest code + all the readonly data (e.g. strings) + windows UI components + misc, overhead
Heap size is just one of the memory modules for any managed process (.net or java). That is why you will see that iTest process will look like taking more memory than just the heap size. But if heap is full, you will have problems with iTest running properly even though the underlying operating system has memory available.
On the other hand, if the underlying operating system is running out of memory, increasing iTest's heap size will only make the matter worse.
The same thing will be true on Linux. Only difference is people do not run a lot of other programs on Linux and thus operating system has more resources available. But if you run the same set of programs with the same memory usage patterns, you will get the similar slow down in Linux.
If you are willing to see the graph after the test is done, you can write your chart data to a file and plot that csv file in Excel after the test run. This technique allows one to run the test using itestrt or itestcli.
