Pango 1.50 introduced the possibility to use a percentage in the size setting, e.g. <span size="120%">will make the text 20% larger. It is, however, not clear what the starting value is and whether multiple size spans accumulate.
For example, assume the font size is 20pt:
aa<span size="200%">bb<span size="200%">cc</span>dd</span>ee
I would expect the cc to become 80pt (200% of 200% = 400% )size but it is rendered at the same size as the bb and dd.
I suspect this to be an small bug in the implementation of the percentage handling. For example, in the following case, size spans do accumulate as I would expect:
aa<span size="40pt">bb<span size="200%">cc</span>dd</span>ee