The first log backup on Monday is so large because it contains all the log records since Friday afternoon, including all those index rebuilds.

p.s. Stop shrinking your database!!! If you're doing it after the index rebuilds, you're completely undoing all the work that the rebuilds did. If you're doing it before the rebuilds, the rebuilds will have to grow the database again.

p.p.s Rebuilding and reorganising is just duplicating work.