But if Biden had huge crowds 9 months ago and dog shit crowds now, would you think he is gaining in popularity
No, I wouldn't draw anything from crowd size, on it's own. Crowd size will vary depending on weather, local population, pre-speech entertainment, etc. His crowds aren't what they were in 2020 when he was claiming to have 50,000+. Does that mean he's fading? Lunatic Kari Lake was claiming the 100,000 people at the Iowa fair were there to see him, in spite of the fact there were 100,000+ at the fair every day. Trump recently spoke in a gym with a capacity of 9,000, it was at least a quarter empty. He was filling arenas last cycle. Most of the current Trump narrative ITT is driven by Trump's paid social media team. They put some nonsense out, the Trumpers ITT immediately accept it as gospel.
Hillary Clinton was polling over 50% in 2016, there were no real big crowds for her. Bernie Sanders did have some massive crowds in 2016, he was ~10-15% behind her in the polls, and eventually lost. Crowds are hardly an indicator of winning or losing. Facts are, neither is polling at this point. The 2004, 2008, 2016 elections are littered with people (Howard Dean, Hillary, Rudy Guiliani, Jeb Bush) leading in the polling who wound up losing, in some cases losing big.