When my seattube was to narrow from being overtightened, it wasn't properly round.
Maybe check it with calipers. If left to right (as in your picture) it's narrower than back to front, then it sounds like it's over tightening.
@hugo7 started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
When my seattube was to narrow from being overtightened, it wasn't properly round.
Maybe check it with calipers. If left to right (as in your picture) it's narrower than back to front, then it sounds like it's over tightening.