Being half giant means that even when Atsushi is using his magic to make him more or less human sized, he's still much taller than most people. When his control over his magic slips, however, so does his size. He needs to constantly be controlling it and Tatsuya imagines that it would be tiring for anyone, no matter who.
Other options:
FILL: Team Kyoutani Kentarou/Yahaba Shigeru, E
sexual content; half giant!Murasakibara
word count: 1268