法華經義記 1.54

1.54

「摩訶迦葉」者，摩訶言大，迦葉是姓，此翻為龜也。所以呼為大者，異不三小迦葉，故呼此人為大也。

Regarding "Mahākāśyapa": 'mahā' means 'great', and 'Kāśyapa' is a clan name, translated here as 'turtle'. The reason he is called 'great' is to distinguish him from the three lesser Kāśyapas; therefore this person is called 'great'.