I have a table with about half a million records using about 250M on disk, and this is just a small dev database - I have much bigger ones in production. If I click 'change' next to any field in this table (or others of a similar size), phpmailer crashes with an out of memory error. I've tried it with a PHP memory limit of 1Gb, but it still happens. It looks like it's trying to do something dumb like read the entire table into an array to figure out what to display for the field editor page. It works fine for smaller tables.
I attach an example screen shot.
Adminer has no problem doing the same thing.
I'm running latest from git (4.0beta2) on PHP 5.4.12 on OS X.