Not so sure about the "padded" part, but yes, ST novels, especially from that era, were (and to some extent still are) intended to feel like episodes.
I don't think that's true at all. Back then, reading was a far more popular pastime than it is today, and TV was still considered a less respectable entertainment medium than prose, so if anything, it would've been seen as preferable for a book based on a TV show to feel like a book, rather than limiting itself to what TV was capable of. There's a lot in the Bantam novels that felt different from TV episodes -- the stories were often more epic in scope (e.g.
Spock Must Die! or
The Starless World), there were sometimes long narrative passages focusing on ideas rather than depicting "onscreen" events (e.g. all of
The Galactic Whirlpool's worldbuilding and historical asides), and so on. Part of what made
Planet of Judgment so interesting was that Haldeman
didn't merely imitate how TOS did things but used his experience as a military veteran to depict how landing parties
should have done things instead, with body armor, more detailed emergency procedures and strategies, etc.
The reason Bantam Trek's editor Fredrik Pohl hired veteran SF authors like Joe Haldeman and Gordon Eklund, as well as fan authors like Marshak/Culbreath and Kathleen Sky, is because he wanted books that felt like SF novels, even if they weren't necessarily that faithful to TV Trek. The primary goal was to sell books, period -- not just to fans of the show, but to SF readers in general. Note that the original covers did not feature the
Star Trek logo -- they just gave the title of the book, with small text identifying it as a Trek novel. While Trek was growing quite popular in syndication, it wasn't yet the institution it later became, so the goal was more to sell Trek books to a general SF readership than to pander exclusively to the Trek fanbase. The books weren't short because they were trying to feel like episodes, but because it was typical at the time for SF novels to be shorter than they are today.
As for the modern novels, I for one have rarely tried to make my books "feel like episodes," because the whole point of doing stories in a different medium is to take advantage of that medium's distinctive strengths, not merely to imitate what the original medium already does. I know that Marco Palmieri, the editor of the DS9 post-finale novels, resisted the tendency of fans to treat the books as "Season 8," because he strove to embrace the freedom of prose to tell stories in ways that TV episodes couldn't.