Kurt Van Meerbeeck points out the fact that SGA re-sizes are occurring after an upgrade to 11.2 despite the fact that automatic memory management (AMM/ASMM) is disabled via the MEMORY_TARGET and SGA_TARGET parameters being set to zero.
It turns out that this is an expected behavior in 11.2. From Oracle Support note 1269139.1:
This is expected behavior in 11.2 for immediate memory allocation requests, which added this as a new feature when automatic memory management was disabled.
Ignore the resize operation which is performed to prevent an ORA-4031 error from being raised, or set the parameter _MEMORY_IMM_MODE_WITHOUT_AUTOSGA to false in the instance to disable this feature with the consequence that in future an ORA-4031 error would be raised.