A few months ago I (M21) bought my girlfriend (F22) a much larger dildo: https://www.beate-uhse.com/realistische-hautfarbener-dildo-20-cm-p-25701/I only have about 13 cm. This has never been a problem in that sense and she always said she was more than happy with me and the sex. But she cums me relatively rarely; I can count on one hand how many times it has happened. Since I've been having cuck fantasies and watching cuck porn for some time, I got over myself and bought her this much larger dildo.
At first she was very reserved and almost didn't want to touch it at all. It was her first dildo. I guess it just looked too big for her, although I was hoping that she was secretly curious. At some point I tied her to the bed during sex and blindfolded her so that it might take some of the tension out for her if she didn't have to see how I react to her reaction to the dildo. I then asked if I should try it out on her and she said yes.
I slowly inserted the dildo into her with lots of lubricant. It made me very horny watching this much bigger cock enter her. A little over halfway through I stopped and started fucking her with it. She immediately started moaning a lot and making noises that I hadn't heard from her before. Over time I was able to go harder and deeper into her. In the end it was almost all the way inside her. Soon she came just from the dildo PIV, without a vibrator or anything else. I had never managed that before, and with the dildo this happened on the very first try.
Since then, I've been able to watch her ride and cum on it many times, but also use it on her, I really enjoy doing both. After it was inside her, she's always so stretched out that I can hardly feel anything when I'm inside her, which makes me very horny. We have spitroasted several times, I love being in her mouth and feeling her moan on my cock.