the memory required for setting of user log cache size is related to the number of user connections
if you have configured a lot of user connections, then this amount of user log cache needs a lot of memory
why it's working in your 15.x version, no idea
btw. I don't know your app, but that's quite a HUGE size for a user log cache
the user log cache is flushed as soon as your transaction completes, you might be wasting a lot of memory