My understanding is that Cumulative Updates contain every update that rolled out before it. This means that if you are running CU2, but CU6 is available, you can just install CU6 and get all of the updates from CU3, CU4, CU5, and CU6.
With this new model, however, won't CU size grow enormous? SQL Server versions last for around a decade, so maybe by CU100 the size would be unmanagable?
Could I get general thoughts on this concern? I'm wondering if maybe just my understanding of the new system is wrong? Thank you in advance.
SQL Server 2017 came out two years ago, so you can just check the file sizes. The answer is no, in fact they are smaller than the old model.
If you check the list of CUs you'll see they grew from 250MB for CU1 to 520MB for CU17.
If you check the list of SQL Server 2016 downloads SP1 which came out in 2016 was 550MB and SP2 which came out in 2018 was 774MB. The 2016 CUs are at least 500MB. Even the first SP1 CUs were at least 400MB.