It's the click of the switch that is stiff, they don't use Omron but some other square switch, it seems the same in SS R310 and S310 as well, same square switch that is stiff to click. It's lower than Omron I guess that's why they all are adopting it for middle click
G603 photos, it's poor quality the buttons are separate the photos cannot show that much detail with the Logi small buttons spacing hell even with your own eyes you can barely see it on a G403 compared to SS 310s.
G603... smaller? I don't know, make me a G4003 a much bigger one especially length wise for the palm. There are tons of tiny mice under 120mm to choose from as well as middle size 124-128mm but close to none 132mm+.