It is just that you did not increase the min-height enough for two lines.
Also, given you are causing more space on single lines, you may want to decrease / remove the bottom padding.
Change the min height rukle to the one below
.wp-caption p.wp-caption-text {min-height:45px;padding-bottom:0;}
You can further reduce the vertical space with
.wp-caption {margin-bottom:0;}