I am jumping back in to point out that the "shorter" picture book "trend" goes back decades. Look at two classic picture books, Where the Wild Things Are and The Snowy Day, both published in the early 1960's as examples. Each is just a little over 300 words. Such books weren't recently invented. Typically such picture books are author-illustrator creations, though not always.
The long-term trend I DO see is this: As the children's trade market gradually shifted from libraries to bookstores in the 70's, 80's, and 90's, publishers increasingly had to cater to parents, who wanted kids to read "real" books. Parents want early readers and early chapter books. The picture storybook -- the format for those 1,000-word and up stories--has become almost extinct, with only Patricia Polacco able to publish in it anymore (perhaps that's a bit of an exaggeration).
Are there actually more of short PBs being published today than 50 years ago? I don't think anyone has actually done the research to determine that, but that has become what we all believe. For what it's worth, I see "longer" PBs, maybe 400-700 words, written by an author and illustrated by someone else, still being published. If you're a writer, look to those as mentor texts, not ones created by an author-illustrator.