Jason Derulo Says He Was Only Semi-Aroused in Deleted IG Post

Jason Derulo says he's a victim of a social media double standard -- and he informed us about this while totally gloating about the size of his junk. Well played, JD.

We ran into the singer Wednesday hours after Instagram deleted his famous underwear pic by a pool in Bali -- the very definition of living large. IG's explanation was that Jason was showing off "aroused genitalia" with the giant bulge in his briefs.

Jason's throwing cold water on that theory though ... telling he wasn't all THAT aroused in the photo, calling it more of a "semi" sitch. He says basic knowledge of the male anatomy supports his claim. Buckle up for Prof. Derulo's lesson, class.

Now, as for any potential shrinkage -- he was emerging from a pool, after all -- Jason admits that was NOT an issue.

Yeah, as they say in ancient Rome ... res ipsa loquitur. The thing speaks for itself.

Watch the clip ... there is a serious point Jason wants to get across -- he believes IG is discriminating against his natural blessings. He's demanding equality, and he's ready to chat with Facebook and Instagram about it.