Per this article I thought Renoirs were kind of a big deal. If so $ 100,000 seems a little low if he was a major figure.
IANAAH (I am not an art historian), but this sentence from the Wikipedia article on the artist might be a clue, “A prolific artist, he made several thousand paintings.” In other words, his paintings aren’t terribly rare.
Also, 5x9 isn’t very big – about half the size of a standard piece of paper. It’s not like this is a major painting.
Plus, it does depend upon how the bidding goes on that day.
Perhaps there are two people just dying to lay their hands on an original Renoir - it is not uncommon to see a bidding war where this could quickly raise the price.
As an Antiques Roadshow fan, I am often amazed at the low price of many paintings/sketches found from famous painters. But they too mention that size of painting comes into play, as well as style - if the artist is famous for lush landscapes but you have a portrait of an old man, it might not be worth as much.
For sure being small makes a difference. It’s not going to dominate an entire wall in your living room or be the crown jewel of a gallery exhibition. Having a muddy provenance probably plays a role, too. Sold to a collector then disappeared for 87 years isn’t going to raise the price as much as being sold 8 or 10 times for continually escalating amounts.