When we say that it is appropriate for a man to taste all the foods cooked for Shabbath, before Shabbath (see http://www.atorahminute.com/2017-12-22 ), obviously this does not refer to eating large amounts. On the contrary, one must be careful to only eat a small amount, for the purpose of tasting only.
If one were to eat a large portion, it would no longer appear that he was eating the food in honor of Shabbath, in order to ensure that it is prepared correctly. Instead, it would appear that he is eating it for his own pleasure.
If Friday is a fast day and one is not permitted to eat, the Eshel Abraham says that one should taste and then spit the food out. Hakham Yosef Hayyim, 'a"h, says that even if one spits out the food, one will still merit to receive celestial life, as Rabbenu the Ari, z"l, mentions. Therefore, even on a fast day one must be particular to taste the food to ensure that it is befitting the Shabbath. If it is not a fast day, however, one should not spit it out but should taste and swallow.
(See Meqabsiel, 2nd year, Lekh Lekha, Oth 14)
Hilchot Shabbat, Shabbath, Hilchos Shabbos, tasting food before Shabbat, Fast day