Big tractor isn't more complex than smaller machines. It's just bigger. So you're essentially having 1000x the failure rate.

Big farm will also have more than one machine and the "downtime" will be mostly scheduled maintenance, very little chance of your equipment just deciding to not work today. In case of many smaller farms, well, you can ask the neighbour... from what I remember from living in small village with many farmers "harvest" was often few farms pooling their resources and doing all the fields together, instead of each doing their own.