In my experience it isn't the web developer / designer who makes this decision. It is either a) their boss, who grew up in a world of print or b) the client.
I have had both happen to me, more often than not the client. I have had a client phone me up before saying "this doesn't look like it did before" and eventually I find out they have their font size enlarged so that boxes don't line up the same way they did before, or scroll bars are present that weren't before, but still completely usable. I explain this to them, and their answer is simply "fix it". Even though they themselves are using an increased font size to help them read, they want to force their dream aesthetic on everyone who visits their site, usability be damned.
I have had both happen to me, more often than not the client. I have had a client phone me up before saying "this doesn't look like it did before" and eventually I find out they have their font size enlarged so that boxes don't line up the same way they did before, or scroll bars are present that weren't before, but still completely usable. I explain this to them, and their answer is simply "fix it". Even though they themselves are using an increased font size to help them read, they want to force their dream aesthetic on everyone who visits their site, usability be damned.