Yeah, that is strange. This issue has existed in the past in previous Field Map versions and Collector prior to that. I saw the post that says it has gotten a lot better in recent releases.
What results do you get if you look at the same map on multiple mobile devices? Do they all reflect the same size symbology? And what about other published maps, have you tried changing symbol sizes on the source map and republishing, then seeing no change on the mobile devices?
Try changing the symbol entirely, republish and see if the cache is really being cleared. I've also had to reboot the mobile device in certain odd circumstances to eliminate some unexpected behavior. Or physically reload a different map then switch back to force a reload and calls back to the data server.
It can be quirky. Keep chasing it.