The heading is general, and cover various areas.
EG:
When editing a file with Circle elements, Inkscape adds *seemingly* superfluous Sodipodi attributes to it:
<circle id="circle3916"
style="fill:#ff8080;stroke:none"
transform="translate(0,1710)"
r="19"
cy="90"
cx="45"
sodipodi:ry="19"
sodipodi:rx="19"
sodipodi:cy="90"
sodipodi:cx="45" />
A circle element has only one radius, as far as I know. There is a separate Ellipse element.
So, why the extra attributes? And can they be safely removed to make a lighter file?
Other issues is bloated Style attributes (see bug#170184) and new in 0.46 Inkscape adding Perspective to everything, irrespective of whether the 3D tool is used. One default I could deal with, but it keeps adding more.
Example of the former:
style="opacity:1;fill:#000000;fill-opacity:1;fill-rule:nonzero;stroke:none;stroke-width:0.30000001;stroke-linecap:round;stroke-linejoin:round;marker:none;marker-start:none;marker-mid:none;marker-end:none;stroke-miterlimit:4;stroke-dasharray:none;stroke-dashoffset:0;stroke-opacity:1;visibility:visible;display:inline;overflow:visible;enable-background:new"
added to every damn element in a large drawing blows the filesize right out.
First, remove Default values where they don't need to override inherited styles;
Second, remove redundant styles, like Stroke styles when Stroke is none;
style="opacity:1;fill:#000000;stroke:none"
Third, have an option like in Illustrator where you can define stylesheets in the header and reference them for often-used styles, eg if long styles are needed on every element
As a Web standard, size is an issue in SVGs.
Oh, and opacity:1 in the above example is also redundant. The above long style recurs 131 times in one of my drawings. Removing the redundant styles with a search-and-replace shaved 42kb off the file.
It also demonstrates the problem of rounding errors, a separate issue, where setting 0.3 becomes 0.30000001.