When a large transaction is committed, ObjectDB increases some internal structures, i.e. allocates more memory, but from that point the structures remain big and the memory is not released even if all the next transactions are small.
This problem was found during a heap analysis (see #9 in this forum thread), and was fixed in build 2.4.3_02, but the fix was removed later in build 2.4.3_03 because it caused other problems (#10, #11 in that forum thread).
ObjectDB Support