The Mishna quotes three opinions on this matter:
According to the Tanna Kamma, the worker can eat as much as he wants.
Rabbi Elazar Hisma rules that he cannot eat more than the value of his daily wage.
The Hakhamim say that a person would be allowed – in theory – to eat as much as he wants, but we teach workers to eat a reasonable amount so that they will not have trouble finding work in the future.
The Gemara also quotes a discovery made by Rav, who found a megillat setarim in Rabbi Hiyya’s house, where the opinion of Isi ben Yehuda appears. Basing himself on the simple reading of the passage in Sefer Devarim (23:25) Isi rules that anyone who passes a field is allowed to eat from it. Rav objects to this ruling, saying that no farmer would be able to function under those circumstances.
A megillat setarim – concealed scroll – is a collection of notes taken by students during Talmudic times. For generations it was accepted practice that Torah she-ba’al peh – the oral tradition – was not set in writing, and only later was writing allowed due to difficult circumstances (i.e. there was a fear that the oral traditions would be forgotten). Nevertheless, students did take notes on the lectures and discussions for their own use. Since these notes were not publicized, they were referred to as megillat setarim. According to the ge’onim, they were not really hidden, rather they were simply not available to all. We find a number of places in the Gemara where Rav quotes teachings from Isi ben Yehuda that he found written in this megillat setarim.