It's entirely possible the effect was more subtle - the designers could have been operating with a slightly retro mindset. But surely they were going by the script which stated Excelsior was new, fast, and generally awesomer, and Oberth was a midsize science ship?
Just re-watched STIII, and as much as I want to see the Oberth as an older design, I don't think that was production intent. The movie tries to sell this idea that the Enterprise is outdated now. The Excelsior gets all the attention there, but I suspect they made the Grissom as a more streamlined design similar to the Excelsior specifically to make the Enterprise look like an old relic in a Starfleet that was leaving it behind.