Page feels slow, circle instead of my mouse, the screenshot of M3 expressive shows less space for content and recipient address but the send button is clearly easier to find
No but you see, they did eye-tracking tests and users "find" the send button in 0.8s instead of 1.6s, so it's clearly worth it to reduce the space for content even further and add even more enormous amounts of whitespace. This is science you guys!
Btw: extrapolating an exponential growth rate for the amount of whitespace in modern UI I predict that smartphone screens will consist entirely of whitespace before 2030.