Jon is small, Martin is big. This doesn't really impact much besides making kissing a little inconvenient. But then Jon starts noticing it more and more. Maybe it's how easy it is for Martin to pick him up, or how when they hold hands Martin's engulfs his. He starts thinking about how easy it would be for Martin to hold him down, grab both his wrists in one hand. How big Martin's fingers would feel shoved in his mouth, around his neck.

Basically, Jon asking Martin to manhandle him, just really have his way with him while they fuck, and Jon enjoying how small and fragile and safe it makes him feel.

Bonus Points:
+Begging
++Martin tentatively getting rough
+++Spanking