RPMs will not drop while rolling?

I recently replaced the throttle body on my car, because the old one was worn out. The only one I could find that wasn’t almost $400 were aftermarket ones that were a bigger bore than the intake manifold (70mm—>64mm manifold). according to the company that sold it to me I didn’t need any extra parts or a port match. After installing the new throttle body, I noticed that my RPMs would sit at about 2,000 to 2,500 only while rolling in neutral and once my wheels completely stopped and my speedometer 0 MPH would it go back to idle (about 1,000 RPMs). Is this due to the difference in bore sizes or is there something else that might be making do this?