Reply to:”The Gemara in Shabbos (75a) says that according to Rabbi Yehuda, one who extracts the dye from the chilazon on shabbos is chayev for Dosh. Now the minimum shiur to be chayev for Dosh is a Grogeres. The amount of dye taken from a Murex Trunculus is less than a Grogeres.”
Acc> to the Raavad it’s talui in the chashivus not the volume which makes disha of a chilazon a chiyuv chatos.
The Rabam shtims with the shita of Rabeinu Avrohom ben HaRambam that petzia is the sepraration of the chilazon from the shell thus involving a kigrogeres in volume as well.