Mr. Health,
If you look at Rabbi Bodner’s sefer (Visen Bracha), he quotes from R’ Shlomo Zalman as follows. In E.Y. where people eat pizza just for Seuda purposes the Bracha is Hamotzi even on a drop, In America however where people sometimes eat it as a snack it depends on one’s Kavana (Meaning if your Kavana is for a seuda you make Hamotzi even on a drop,if your kavana is for a snack then you only have to make Hamotzi if you eat the Shiur Kovea Seuda) . I didn’t see the Teshuva from R’Moshe you mentioned, but it’s possible that that’s why he holds in America it’s Mezonos (bec. he holds eats mainly eaten for a snack) not bec. of the Shitas Hataz.