Still in <home>/Library/Mail, but hidden
Mail still appears to store messages in <home>/Library/Mail, but the files are hidden from the Finder.
If you open your Terminal, you can do "cd Library/Mail" and you'll be in the directory where the messages are. "du -sh *" will show the size of each file or folder in that directory; "ls -al" will list the files and folders.
Good luck.