I have similar problem with my Ducati Monster 695 that I also removed and after a hour or a bit longer didn’t fit anymore.
The culprit is ethanol. This is the government mandated stuff that screws up plastic fuel tanks. I was able to fix it by emptying the fuel tank and put it out in the sun for a bit and all shrinks back to normal. Not forget to move the fuel tank around so not only one side is exposed to the sun…
Many plastics are susceptible to chemical reactions from various types of fuels and additives. For that matter, so are metals. Steel tends to be good container for most petroleum products, but alcohols can cause oxidation. Steel tanks are the safest bet, but they can be heavier and require special internal coating to protect them from alcohol and water (ultimately oxygen). Plastics are lighter and may be resistant to oxidation from alcohol but be more susceptible to chemically reaction in other ways such as swelling, shrinking, softening or hardening.
There are many types of chemicals – not just ethanol – that are used in fuel. Butane can be used as a temperature stabilizing agent. Alkylaminants and alkyl phosphates are detergents used to reduce the buildup of contaminants in the combustion chamber. MTBE, ETBE, ethanol and biobutanol have been used as oxygenators, although MTBE has been widely removed, due to environmental concerns. White gas, or ‘pure’ petroleum gasoline, consists of a wide range of hydrocarbon molecules and impurities such as were derived from petroleum or biomass, each chemical has a different molecular structure that could react to certain plastics. Alcohol or ethanol may not be the only problem.
I’ve seen white gas cause swelling or softening of certain plastics and rubbers. Try to hold gasoline in Styrofoam sometimes. All manufacturers are quite aware of alcohol, ethanol and petroleum chemicals in fuel, and would not intentionally use a material that was prone to failure, although they are challenged in finding a single material suitable for all blends of chemicals used worldwide.