The desktop pager on the taskbar is showing extra spaces for each workspace. It used to show only one tile per workspace. I am using Lubuntu 18.04.
How do I fix this?
In Lubuntu 18.04 LTS (and Xubuntu as well), when a second monitor is connected, the workspace tile for each workspace expands to account for the second monitor. That is the "extra spaces between workspaces" that you observe.
Try enabling both monitors and maximize a window in each one to see the effect. The windows will appear side by side in the same workspace tile. If you disable one monitor, the workspace tile will revert back to its normal size.