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By default, there is no limit to how much of the database is cached, with the data never being purged from core and the memory consumption increasing with the recorded data.
Interested applications may limit the size of cached data. The limit refers to unmodified database blocks only. It does not include modified blocks, data padding coming from whatever source (like memory allocation), supporting data structures, etc.
Included with the supporting data structures is the file blocks index, which owing to suboptimal organization and being always cached may take quite some space.
The modified file blocks are not purged from memory, not until the transaction is closed and data is written on disk. For this reason, large transactions may see large memory use.