for comparing members size?
The member list for a load module dataset will give you the member size. It will give you also the TTR. The TTR is an offset from the beginning of the dataset. If you replace a member, it's TTR will change. If you capture the member list periodically and then compare with the prior generation, you should be able to figure out which members changed.
note: If you compress a PDS, then likely the TTRs will change, giving many false positives.
Use the LMMLIST service of ISPF to get the member list information.
note: if someone uses AMASPZAP to alter a load module, neither the size nor the TTR will change (nor your eyecatcher dates). I do not think you can detect such a change without saving the entire load module and doing a compare.