I bought a Ventris earlier this week.
Same problem with the coupler.
The smaller body tube is 1.948" id while the coupler is 1.872" od.
The other end isn't much better: bt id is 2.458" coupler is 2.42".
It is clearly the wrong part for the kit. As pointed out above, there is no easy fix other than replacing it with a custom made balsa transition or a paper transition.
I emailed Estes two days ago and still have not heard back, which is not the norm for their customer support based on past experience................
My life's goal is to be an idiot savant. So far, I'm only halfway there.