As you already insinuated, the J2EE instance needs a lot of memory. If you can delay its installation, the ABAP stack will temporarily have more memory at its disposal. However, if you install the J2EE instance later, your end-users might complain as the system might be less performant as it was without the J2EE instance. Installing both systems at the same time sets end-users expectations right from the beginning.
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