Zoom isn't the right term. The vertical resolution is unchanged. The image is being stretched horizontally to undo the anamorphic squeeze. When cropping to 4x3, there is less horizontal resolution only. Film grain then gets stretched and appears bigger and splotchier, which contributes to the drop in picture quality.
But that's exactly the way the compositors describe what happens when you're dealing with 4x3 vs 16x9. Take the discussion on why TNG can't be shown in widescreen,
Eric Bruno: You would have to really significantly change the composition of the shots. Why don't you just push into it (Zoom in)? Well, if you push into it, whatever information you have going on at the top and bottom is gone. So you actually have less information than what you had in the original. And also on top of that, you're going to be softening the image.
And that's what we get with these shots, they pushed into the image which resulted in the shots coming out soft.
My guess is that they shot these scenes anamorphic while the regular shots that dominate the show were not shot anamorphic. I may not have that right, but there has to be a reason why these shots look soft when every other shot does not.