The gemara raises the concept of מי איכא מידי דלישראל שרי ולעכו"ם אסור, meaning that goyim can't be frummer than jew - if it's assur for a goy, it has to be assur for a jew. Tosafos raises various questions and exceptions to this rule. Tosafos writes that anything which is a mitzvah for a Jew can be an issur for a goy, such as shabbos and studying Torah are assur for a goy. The rationale is that since the entire concept of it being assur to a goy stems from the fact that it is a mitzvah unique to Jews, it doesn't break the rule of מי איכא מידי וכו. At the end of Tosafos, they write:
אע"ג דעובד כוכבים אסור במשהו בשר וגידים ועצם, וישראל לא מיתסר אלא בכזית בשר, הא אמר ריו"ח דחצי שיעור אסור מה"ת
Tosafos seems to imply that there is no concept of a shiur when it comes to a goy, so whether they eat more than a kezayis or less than a kezayis, it is all the same issur (even though by Jews it is more chamur to eat a kezayis, but since there is an issur of chatzi shiur it doesn't violate the מי איכא מידי concept). This seems to support the idea that shiurim are a halacha l'moshe misinai that was said to Jews and not to goyim. However, the Pri Megadin understands from Tosafos that by goyim even the gidim and bones combine with the meat to create a shiur of a kezayis, but the requirement of a kezayis to violate applies to a goy as well. The language of Tosafos where he contrasts that a Jew is forbidden to eat a kezayis of meat, implies that by a goy the bones combine to create a kezayis. However, the Rambam (9:10) writes explicietly that shiurim were only given to Jews, but Goyim are in violation for even eating the smallest amount of eiver min ha'chai.