I can answer this question. I've had lovers who I was seeing simultaneously (not literally in the same room at the same time, though that would've been hot, but I would see them within a day or two of each other frequently and there were at least one or two times I saw one the same day as the other), one of whom easily fit into a normal sized condom and one of whom absolutely needed an XL sized condom. I experienced absolutely no change in sensation with Mr. Normal after being with Mr. XL. Not an issue whatsoever. The vagina is very elastic -- after all, women can enjoy being fisted and then go right back to enjoying PIV sex for gosh sake's!
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.