Question

why do so many families of farmers cultivate such a small plots of land

Answer 1

it's because large farmers buy huge land with their money which the small farmers are not able to do so.and the large farmers have ample amount of money to buy fertilisers, pesticides to make the crops grow faster but the small farmers don't have money to buy such things so their crops grow comparatively low so they work in the lands of large farmers to get money to run their family but they get very low wages which is not enough to grow crops in their field even to meet their family needs.
coming to the point 》low caste people don't get large plots of land

Answer 2

Some farmer do not have so much land so that they cultivate for their own and complete their livelihood so they work on other big farmers land so that they can fulfill their basic requirements.