No it isn't possible.
Testing things like memory consumption and performance should be done on the device, not the simulator. Even if you could limit the memory in the simulator you wouldn't get the same effect as there are very many different factors that will dictate how much available memory any given iPhone device will currently have.
You can simulate a "memory warning" to test if your responses to that warning behave correctly, but other than that, you should really be testing this kind of thing on a physical device.
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