In SVG output, units are different from other outputs
While all output formats treat opengl coordinates as "points", the svg treats them as "pixels". This makes svg figures 25% smaller than other figures.
This issue has already been reported on the mailing list (see http://www.geuz.org/pipermail/gl2ps/2016/000451.html) and there is also a proposed patch to unify the concept of units for all outputs through the use of a new "resolution" parameter.