Lines 60 and 116 of /my/index.php [1] hardcodes the min-max values for the column widths. It should be using $THEME->block_l/r_min/max_width settings. This is what other pages like index.php [2] do.

[1] http://cvs.moodle.org/moodle/my/index.php?annotate=1.28#l60
[2] http://cvs.moodle.org/moodle/index.php?annotate=1.233#l37