Food Blessings

The Gemorah in Berachos (35) says that one must say a beracha before benefiting from food for the land is owned by Hashem and it would be like mielah to eat without a beracha.  From there one derives the obligation to say a beracha before eating.  How do we know an obligation to say a blessing after eating?  Rashi explains its derived from birchat hamazon.  We see from here that the two types of berachot are distinct; the beracha beforehand is to remove the issur of benefiting without a beracha, however the beracha afterward is merely to give thanks and blessing for the food eaten.    

Tosfos (39a) says that even if one eats a minuscule amount of food there is an obligation to say a beracha beforehand.  However, regarding the after blessing one is only obligated if they eat a כזית.  What’s the difference?  We see the same principle; the blessing beforehand is a מתיר to allow one to benefit from the food and applies to any amount, on the other hand, the blessing afterward is an obligation of thanks and blessing that only kicks in when eating a respectable amount.

Based upon this idea we understand why there is a ברכה כללית of שהכל for the blessing before eating but there is no such beracha for after eating.  The difference is that the beracha beforehand is to to remove the issur so that is satisfied by a general blessing.  However, to give thanks for the food eaten, the beracha must be specific in order to give proper thanks.

Addition from my father: The Rabbenu Yona Berachos (27b in dafey haRif) and the Kol Bo siman 24 bring an opinion that on less than the shiur one says a ברכת שהכל, not the specific beracha of the food.  Why is this?  Rav Warman (Shearit Yosef volume 1 #2) explains that they hold even in the beracha rishona there are two elements of obligation; the specific beracha is only for the element of thanks and praise which one is obligated in only if they eat a substantial amount, it is a shehakol that suffices to remove the issur of benefiting from the world.