Hey OP,
Guy here. I know, not your target audience, but I still hope what I say might be somewhat insightful to you.
First of all - I'm on your side. You guys sound like a great couple and I hope you can work this out.
This general type of story plays out a lot on reddit and honestly, usually I take the guy's side. But your story is very clearly different from most of them. The big thing, to me, is that you totally honest love and lust after your boyfriend, and find him sexy as fuck, and, in your heart-of-hearts, do not think you are settling for him in any way. Of course I have only your words to form that opinion, but that is the picture that they paint.
(On the other hand, most iterations of this type of story boil down to "girlfriend is much less turned on by current boyfriend compared to bad-boy/slobby/asshole ex, but she feels that sex is only a 'small part' of the relationship anyway, and they much prefer current boyfriend because he treats them much better". In that case, I fully support the right of the man to find a relationship where the girl finds him just as sexy and desirable as he finds her. Again, this seems fairly different from yours)
Anyways, I'm not saying you've done anything wrong (minus the part where you told him you were thinking of the massive dong - we'll come back to that), but I want to hopefully give you some insight into what he's thinking.
And to be clear, given what you're saying, what he's thinking (that you aren't attracted to him compared to someone bigger) is not necessarily correct, but it doesn't come from nowhere.
As people have pointed out, we live in a society where "bigger is better" is basically everywhere. Small penises are the butt of like, half the jokes about guys (ok not literally 50%, but you know what I mean). "Hot" celebrities brag about their love for, and ability to attract, well-hung guys. Have you watched porn? How many sub-6 inch penises have you ever seen in them?
All of the above is obvious, and you knew all of that. But the most insidious thing, I think, is something that's not often mentioned. Not only is the message "bigger is better", but the companion message "women always prefer better, and anyone who says they don't is lying about it".
For the record, this is totally wrong and stupid, but a lot of guys fear this to be true. There's definitely a confirmation bias here. Everyone hears the story of the girl who thought that size didn't matter, and then had like 20 orgasms with a coke can and now is hooked. While there are just as many stories of girls who didn't hated sex with the wrist-thick dick, but realized they were happier with smaller or normal-sized penises, they're much more quickly forgotten about.
This leads to some perversity. Your boyfriend thinks that 1) you love him and care for him and 2) you are not an asshole. These are good things (really!). But, if he thinks these things are true, but also that girls always prefer huge dongs, he'll think that you will usually lie to him to not hurt him.
So - when you joke about thinking about the massive member when you're about to have sexytimes, or admit to being more vocal with your former fling, he interprets this as "ah she's slipping. This is the truth, that someone made it past her 'niceness' filter".
Because you have to admit - this type of thing DOES happen. A girl might honestly love her partner to bits but wish that he was bigger - and that is just what she would do. She wouldn't tell him honestly "I love you but you're a little small for me", because that's devastating to hear.
Wheeeeew ok having said all that - again, it really sounds like you love the fuck out of your boyfriend and think he is perfect at knocking your socks off in bed. I hope you can keep being empathetic, BUT I see how this shit can get old. At some point, it will come down to whether he believes you or not when you say that you think he is a fantastic match for you sexually. And if he can't... =/ it sucks to say but at some point you'd be justified in thinking this is a dealbreaker.
One thing though - in your other comments you're kinda dismissive about admitting to being more vocal with the ex. Maybe you took exception to their tone and their implication that you didn't actually love fucking your boyfriend - but I hope you see that this is kind of a real thing. Honestly, when I'm fucking my awesome wife, a cue I use to determine how things are going (and what's working fantastically or just ok) is how vocal she is. Sound is, after all, a big part of sex.
Anyways, good luck!