I always wondered, if Loki is a son of Frost Giants, how come he is not a giant himself ?  Sure, he had sex with Angrboda, who is a giantess, but how could he do that if he is not bigger than Thor, who is like an ant to Frost Giants ?

Thats a good question for sure.  The size of the giants gets mixed up.  The gods themselves were towering above the trees.  So large as to block the sun.  The giants were obviously taller than them, but not so much.  Several Aseir took giant women for wives.The true Allfather of the gods Buri ,  took a giantess as his wife.  Loki  was a freaky shape-shifter.  His children  assumed the shape-shift of  their parents.  The horse that Odin rides into battle, Slepnir (the eight legged stead) is also a child of Loki's.  He changed into a hot mare to prevent a giants workhorse from completing a task that would bind Odin to the giant.  The comic version of Loki is not accurate myth.