Originally Posted by
angrysparrow
Partial underquilts are mostly driven by the fact that they are often carried backpacking, where both weight and bulk are concerns.
For a long time, almost all underquilts were full-length. And quite a few manufacturers still make full-length ones (JRB, for instance). But when Warbonnet introduced the Yeti (first commercial partial UQ), it slowly caught on for a number of reasons.
Partial underquilts are generally a little lighter, less bulky, and also slightly cheaper because they use less insulation (down). And while you do add some bulk back to your kit by carrying a pad, that same pad tends to be multi-use (sit pad, windscreen for a stove, etc).
The fit issues of a full-length quilt are mainly a secondary concern. The more advanced radial baffle, shaped quilts do a good job of fitting even if full-length.