That's true for O365 webmail (and can be changed in a single click). In Outlook the Reply and Reply-All buttons are the same size, Ctrl-R is just Reply, you need Ctrl-Shift-R for Reply-All.
Actually 95% or so of my replies are Reply-All, so I wouldn't mind it being the default. I just happen to know when I'd better do a simple Reply and when I should put recipients in Bcc (rarely, as part of my job).
But sure, let's blame the tool rather than the users who still can't grasp a few simple concepts 20+ years after the email entered our daily lives.