The different heights are expected, just like the issue that too wide symbols are replaced with a square symbol..
I've asked about this on the rxvt-unicode mailing list a while ago (Q4 2004), and created a patch, but it was rejected: http://lists.schmorp.de/pipermail/rxvt-unicode/2014q4/002042.html
The patch/branch is available at https://github.com/exg/rxvt-unicode/compare/master...blueyed:display-wide-glyphs. It will make rxvt-unicode not replace too wide symbols with a square icon, but just display them - without special handling. Therefore you will have to add a space after it to display it completely (otherwise it gets cropped).
I've asked to include it into Arch's rxvt-unicode-patched: https://aur.archlinux.org/packages/rxvt-unicode-patched/