Rashi in Pesachim defines the two concepts, and stresses the differences between them:
Rashi on Pesachim 43b:
אכל חצי זית חלב וחצי זית בשר בבת אחת אין היתר משלים את שיעור האיסור לחייבו חוץ מאיסורי נזיר שאם אכל חצי זית ענבים וחצי זית לחם בבת אחת חייב
If one eats half a kezayit of chelev and half a kezayit of meat at the same time, the permitted food does not ...
In the Steipler's Shiurin shel Torah he writes the Chazon Ish's opinion about the amount needed for each Mitzvah. When it is a Mitzva D'Oraisa like Sukkah, he uses 10cm as a Tefach - 60cm as an amah , but when it is a question of a D'Rabannan as seen in the topic of Ma'aka/fence on a roof (in a D'Rabbanan situation b'shas ha'dchak) he uses 9.6cm as a tefach -...
The STAR-K writes
Although ideally a kezayis should be consumed immediately, it is also
acceptable for one to initially eat a small amount of bread after the
brocha and then eat a kezayis of bread within a two, or at least four
minute span later in the meal.