It's certainly true that Labour has the largest party membership. This is from the House of Commons Library:

"As of July 2019, Labour had 485,000 members, compared to the Conservatives, who had 180,000 members. The SNP had around 125,500 members (December 2018), the Liberal Democrats 115,000 (August 2019), Green Party 48,500 (July 2019), UKIP 29,000 (April 2019) and Plaid Cymru 10,000 (October 2018)."

Obviously these numbers don't reflect how people voted at elections. Possibly Labour members have more of an emotional attachment to the "Movement" than do supporters of other parties, and are more likely to actually join the party.

These numbers are tiny compared with a total electorate of nearly 48 million. On the whole, most people aren't very interested in politics and certainly not in joining political parties. Perhaps we get the politicians we deserve.