Question

Why some parts of river bed contains larger stones than others

Answer 1

The others in this thread have already pointed out that the smooth rocks you mentioned are eroded into rounder (though not necessarily round) shapes as they’re transported in the river/stream bed and also as other rocks/pebbles/sand wash over them. I would only add that the more rounded a rock is, the farther it is likely to be from its source (parent rock). Rocks that are more angular are closer to their source.

Answer 2

because of the river bed is made up to heavy and hard rock