When I bought my first bike in 2007, I figured helmet+random crap of storage would be enough
and bought a 35l case. It took a week to realize I was wrong. The only reason I didn't get a
bigger one in the first place was because of how it matches (or doesn't) the bike visually.
Fast forward to today - got a scoot with 60l of storage, and bough top of the line 50l SHAD
topcase. Never even thought twice about it.
Even with 110l of storage now, I could still use the tunnel bag.