Kind of a funny irony in the whole thing.
Douglas Trumbull and his team built the large refit Enterprise for TMP. In subsequent films (not 5), ILM took over the visual effects, and found the model too cumbersome and difficult to shoot. ILM ends up building an off-the-shelf AMT model to supplement the main model. As I recall, ILM wanted to blow up the actual model for Star Trek III since they hated it so much.
Later, ILM builds the 6-foot Ent-D model for Encounter at Farpoint. It's big and difficult to shoot, often having to be mounted upside down. Image G takes over and finds it too cumbersome. They end up building a new model that is smaller and doesn't separate, and easier to shoot.