I first tried to install powerline on my computer and had problems with the arrows not rendering correctly which was easily corrected by installing a patched font. However, although the arrow symbols were rendered correctly they were much smaller than they were supposed to be.
This problem persisted when I switched from powerline to airline. I found that the problem only existed in urxvt. Gnome-terminal rendered the arrow size correctly, but placed the arrows where offset slightly from their proper positions on the line.
Here are the relevant entries in my ~/.vimrc file:
set termfont=Liberation\ Mono\ for\ Powerline\ 10
let g:airline_powerline_fonts = 1
This patched fonts method is a fallback method which works for every terminal, with the exception of rxvt. It is not about the powerline theme, it is about the font size. It doesn't work with 10. Possible solutions can be found here or here.