She is smokin hot dude. I know where you're coming from, even though on a good day(! - I must quit smoking), I'm 8 by 6.
Have you had the conversation with her? I talked with my GF about this on Saturday - though not about size, but about screwing other people (not now but in the future possibly), & hell it included me screwing other women - we're both quite adventurous, & I'd rather me not cheat, & her not cheat, but be a team that plays & stays together.
She said she would find it hard not to have deep emotional feelings about someone who shagged her, especially if they did it well. How unlike most of us guys He He.
It was also pretty clear that though she was interested, she would be totally jealous in a MFF - because she would think I would have the same feelings for a girl as she would for a bloke. I just think that most women feel more secure being the centre of attention if they perceive you as 100% straight - but what's good fro the goose..Lol.
From your post I'd say she must be sexually very responsive, & that this leads you to imagine how much more she would be responsive with a guy as good as you with a bigger dick - & that thought turns you on, and would feel like you giving her a present.
Don't do it! Your relationship as it is would change immeasurably. You are together now, & there is a balance - something like this would cause a new equilibrium to be found, which wouldn't be in your favour, & could lead to a lot of anxiety & a failed relationship.
If you want to fill her up try toys, & - a new one to me from my dirty big pussied love slut - fisting. My fist is more than 12 inches round(average I'd say) but there are no blokes that width. It's kinky, but emotionally connecting - & it's you.
If you must have the thrill - what about swinging? Then you're both getting some, you can watch each other, & she'll be slightly scared of losing you - so the balance of power in your relationship is maintained. She'll eventually get a huge dick, & you'll eventually get some huge tits - though as I said - she is smoking hot you lucky guy!
Hope this helps!