You can never "choose" per-se an app and dictate that the app will have that guaranteed amount of memory.
Sure, Google Maps consume space, but that's for the actual application, does not mean its hogging up memory, Android is pretty smart when it comes to memory management.
When you long press the home key, it shows the recent apps list, that does not mean they are all loaded at the same time. Android keeps track of which activities was shown and if necessary load them again, and once the pressure of RAM space hits a certain threshold, Android will boot them out to make room for other apps.
You cannot override Android's dictation of how and what application should be in RAM, just by a user's presumption that "this certain app should be there in memory and has guaranteed X amount of RAM, because I want it to"
Android is not Windows, where their Task Manager is available at all times via Ctrl+Alt+Del and zap away processes to "free up memory".. That mantra should be firmly repeated ad-nauseum! :)
As Ryan Conrad's statement in the comment, from the discussion arising out of the likes of RAM Boosters, Task Killers et al, this statement sums up exactly why the apps mentioned can have a negative effect on the battery, not to mention introducing instabilities into the system - It's CPU cycles that you should be more worried about than memory. WORD CPU cycles drains the battery regardless and as the commentator says Free Memory is wasted Memory. Let the OS handle it. It is better at doing it. It knows what memory can be freed safely without causing additional strain and drain on the device
Simples really :)