Just posted this response to your Google Group submission, but going to repost it here for anyone who comes across this question in the future:
In addition to the client side trimming (which you already noted), there are a few different server side maximum lengths, defined here: sentry/src/sentry/conf/server.py at 6ba6d960b0e5bcb5e0598f5f8c8f49cba96d3142 · getsentry/sentry · GitHub
The bulk of the trimming is handled within the normalize function of the event manager, if you want to dig in further to identify what’s happening in your specific scenario: sentry/src/sentry/event_manager.py at 6ba6d960b0e5bcb5e0598f5f8c8f49cba96d3142 · getsentry/sentry · GitHub
One thing worth noting is that the trim function sentry/src/sentry/utils/safe.py at 6ba6d960b0e5bcb5e0598f5f8c8f49cba96d3142 · getsentry/sentry · GitHub will track the size of the entire data structure, so variations in the size of the individual members may cause different results in the case of a sequence or mapping type.