Ty, expanding on what Deejay said, if you look at the teeth of the small sprockets you will see two anomalies. First, the dip between the teeth has become wider than it should be, and is not the right shape. This allows the stretched chain to sit in the gaps between the teeth of the sprockets. Second, the tips of the sprocket teeth have acquired chamfers, because the next roller arriving at the tooth each time, is in the wrong place due to having more than the correct amount of pitch, and the wear on the base between the two sprocket teeth is allowing the arriving roller to be even more "late" in arriving. Consequently in severe cases like this one, the tip of the tooth experiences excessive pressure as it tries to pull the arriving roller into position. As the roller moves down the tooth it eventually "jumps" into position to allow it to roll down the rest of the way.
Often the wear on the chain is a bit less than this one, and instead of the sprocket teeth developing chamfers on their tips, they just wear severely down below the tips of the teeth. The teeth thus become more and more undercut just below the tips, until the tips fall off. You then have what motorcyclists used to call a "burned out sprocket": the toothless sprocket allows the chain to jump. I can recall as a small child riding on the back of an old Series 2 Square Four Ariel that had the problem: no matter how briskly the rider engaged the clutch, the bike accelerated slowly and smoothly except for a sort of hissing/clattering sound from the sprocket.
My guess would be that this mower had the sprockets burn out, and the owner replaced them without replacing the chains. This resulted in rapid wear, causing the deformed bases between the teeth, and the chamfered teeth.
