The popularity bars are calculated based primarily on the sales of each track, as well as other internal processes by Apple. So if only one track is purchased off of an album, it doesn't necessarily mean the popularity bar of that track will be completely full. The popularity bar can possibly look similar if you have few or many sales. Popularity bars are relative only to the other songs on that particular release. So if one song is sold and no others have sold, that one song may have a completely full popularity bar.
The popularity bars can also be increased by searches, clicks & preview plays, not just sales.