Many tallits leave the manufacturer with thin, machine-spun tzitzit tied on according to the Ashkenazi tying custom. Many customers opt to upgrade to all-white hand-spun tzitzit strings because they want thicker strings, because they want a tying custom other than Ashkenazi (e.g. Sephardic, Yemenite, Chabad, etc.) or based on halachic considerations. In these cases the machine-spun tzitzit are simply snipped off and we tie on the replacements from scratch.
And of course today many tallit buyers opt for techelet tzitzit.
High-end black-striped and white-striped tallits typically leave the factory without tzitzit.
The difference between thin and thick is fairly noticeable, but now that medium thickness is also available, the difference between thin and medium or medium and thick is a bit hard to discern without close examination.