I would personally permit it, with the caveat that they lose their Magic rating as though to Essence Loss, i.e. they cannot use any abilities tied to magic per the rules on p. 278, SR5. In most cases, this sucks eggs.
I'm honestly just thinking about realistic rules for a windling treetopper pixie attacking an individual who is, after all, five or ten or twenty times their height, and much much more their bulk. Maybe give the difference in the target-size modifiers table (R&G, p.108) as a bonus ...