It seems to be more about Sanae's massive mammaries than anything, as the author note on Pixiv has Reimu apparently yelling "Give it!" or something along that line.
Also, from looking at the other images at the tag, I think Sanae's pair could qualify as [[gigantic_breasts|GIGANTIC Breasts]]. Anyone agree with me?
No, I think "gigantic" is appropriate. If it's the kind of ridiculous proportions that suck any hotness out of an image and just make it funny, like here, then "gigantic breasts" applies, I think.
