...and that also limited features (both functional and security ones).

Of course it is. As I mentioned previously, lower optimization is logical result from faster release cycle. If you are going to optimize in a way that squeezes everything out from limited set of resources (kind of what was done with Symbian), it will take resources (people, time, money) and one has to decide what's the best way to implement features X, Y and Z. Are you going to a) add in resources (money, people), b) add time/push back development of other features/optimizations or c) take advantage of additional processing power etc of the device. At the same time you have to take into consideration the competition which is pushing out new features at very quick pace. This was one thing that effectively killed Nokia mobile phones. Android back in those days was far from being optimized.

Point is, if you want to have both ultimate optimization AND the features to match competition, you will need considerable amount of money and people to do it. Otherwise you are screwed in no time. Jolla has been lamented about poor optimization (which is in many ways true of course), at the same time they have been lamented about lack of features X, Y, Z.... considering their resources those are conflicting demands, you cannot have both highly optimized code AND feature sets WITHOUT considerable financial and human resources OR time to implement it all. Sorry, it's just not doable.

I'd say it's combination of both screen size AND screen resolution, as smaller objects on higher resolution screens are more clear and pleasant to look than on lower resolution screen at the same size (in general, not talking about moving from something like Full HD to 4k on 5" display).