In the repetition of the Shabbat amida, we who are not leading say all lines of the Kedusha, as does the shaliach tzibur.
In the repetition of the daily amida, only the shaliach says all the sentences of the Kedusha. We who are not leading say only the 3 main sentences.
What is the logic behind this?
(I have heard there are other customs, less often followed. Perhaps that is key to the answer.)