Re: Forced perspective, painted backdrops and the refit Enterprise
If you wanted to pass the studio set as both the impulse engine room (in the saucer) and the warp engine room, you could only fit the engine room set as impulse engine room into the saucer believing that the forced perspective / actual set is the real thing "in-universe", too.
Otherwise the "pretended" length of the horizontal tubes would not fit into the saucer, indeed.
"The first duty of every Starfleet officer is to the truth" Jean-Luc Picard
"We can't solve problems by using the same kind of thinking we used when we created them." Albert Einstein