The animals are motivated to do their best to make Animal Farm a success. They modifiy farm implements so that they will be able to use them properly, and they work together willfully instead of being forced to labour and produce.
Another reason for the bumper crop harvest is that Jones isn't around anymore to pillage their goods. However, this isn't the case for very long. When the cows are milked by the pigs, the buckets simply vanish along with the stock of apples. When the animals get back some time later from their labour in the fields, nothing is there.
Squealer explains later that these products are for the exclusive use of the pigs since it is the "brain food" necessary to keep them smart!