It's not so much that Foster wasn't constrained to keep them short as that he was constrained to make them longer, I think. With only three stories per book, he had to flesh them out to reach a novel-sized word count. And of course the last four volumes each did just one episode plus a bunch of original material to pad the series out to ten books.
Indeed. I love the most recent trade reprints (five volumes), as they contain a new serialized essay by ADF about the writing of the "Logs".
When he was asked to make the last four TAS scripts stretch out to four more books (because sales of the first six books had been so good), ADF was able to pull a two-part spec script he'd done for a possible TOS Season Four for "Log Seven". This was the first appearance of Kumara. (I was able to get a message to ADF at a convention not long after the essay came out; no, he can't remember the title of his two-parter TV script, and he no longer has a copy of it.)