The sell bubbles show original "purchase" prices (and dates) paid for all Bitcoin spent by the sellers over the last N days.

The size of a bubble shows a relative quantity of all spent Bitcoin collectively originating from a given purchase price. The BTC amount represented by a given size varies depending on N, the number of days selling is aggregated for.

E.g. if an investor bought a large number of Bitcoin on 31 December 2020 and has sold it today then that would result in a large bubble showing up on the price plot for the 31 December 2020.

If the amount of spent Bitcoin originating from a certain purchase price is smaller than the smallest bubble's threshold (e.g. 250 BTC for N=1d) then it won't be registered on the chart.

Also the bubbles for purchase prices of less than 1 week are ignored due to the fact that coins of this young age are statistically spent daily in big quantities. See Spending Probability charts for more details.