Spent 6 weeks cruising the western isles of Scotland in summer 2018 (the one with the massive high pressure and very little wind), and bearing in mind the lack of diesel availability in that area, I still had no problems with 90l tank capacity. I'm not looking for an argument but why have a yacht if you often motor 500+ miles? Either time the passage to when conditions suit (ie when there is wind) or put up with some time sailing slowly. 60 hours at 5knts is more than plenty for sailors why spend more time actually sailing their yachts, and less time making condescending remarks about "weekend sailors".
People use their boats in different ways and if you want to cruise the Med you just have to accept that you will spend many hours motoring. For example I moved my 37 from Corfu to Southern Spain via Sicily, Sardinia and Majorca. That is over 1200 miles and we had the motor running for approx 85% of the time. That is approx 200 hours (more than twice average UK boaters running time), or 300l fuel. So even with my 150l capacity you can appreciate why we topped up at every opportunity and carried 2*20l cans.
I am not being condescending - I am now a "weekend" sailor - using the term is just shorthand for describing a pattern of usage. You are right that in the UK (as i pointed out) 90l is not really a constraint where the typical annual engine hours is less than 100 or less than 150l pa for a 20-30hp engine. My 37 was on charter for 6 years and clocked up 3500 hours or just under 600 hours a year - and that was just operating in the Ionian with only 2 long passages to and from Athens. That boat had more use in the 10 years i owned it than most UK based boats get in a lifetime.
I agree with you that if you sail in an area with regular winds and relatively short distances between islands or harbours, 60 hours running time is not really a constraint, but please recognise that others don't have those conditions but still want to use and enjoy their boats. Builders recognise this - and in reality most of their boats are sold for use in conditions more like the Med so normally fit larger tanks. The 3 cabin 34 was mostly sold to charter operators where the larger tank is preferred.