When starting area size is set to max the closest resources are in the area where there are endless bitters, like over 1500 m away. With starting area at 100% the first resources start appearing at 500 m. I don't think this setting should affect generation this way.

You are setting multiplier to starting area size to 6x. That means that both resources and biters will start 6x as far.
That also means they will start with both size/richness multipliers and certain conditions like worm presence appropriate to that distance.

I'd say working as designed. Why do you think it should work differently?

It's kind of made my game impossible to finish. I got max biter frequency and size with huge starting area and 10x science multiplier. It simply left me with no resources to get through thousands of biter bases to the first iron expansion :/
I guess I have to put starting area at 1 and lower the rest of biter settings to 2 or 3.

Starting area resource multiplier can be used for cases like this. You can multiply starting resources only to be able to get to heavily defended ones later on.
There are some setups that are potentially difficult/impossible to win. And high science multiplier makes stuff much more costly.
When applying science multiplier (I'm usually using 4) I'll usually apply starting resources multpilier too.

Starting area resource multiplier is nice but the starting patches are still too small (size) support enough belts to keep up with high science, defense and power. I wish this multiplier did not influence so much as richness but mostly size. I remember one time when rso was configured with lua edits I made really nice big starting patches. There was so much config possibilities back then.

Increasing of size is a bit complex topic especially in starting area since it's a limited space that can be hard to place all the patches without many conflicts.
I think it might not use multiplier for starting area size when spawning the ore patches. Not sure if it should - they would get quite far away on 6x.

You can still edit RSO configs they are there in the same way as before to get it just right for your usage. Mod update will reset it back.
Adding starting area size multiplier would be doable but it would need to come with some red letter warnings I think.