Yes, you bought 200gb. But that is not an addition to what was already there, it's 200gb total. If you look at your iCloud settings, it should say total storage 200gb.
You were already using 50gb of storage, so by buying the 200gb plan you added an extra 150gb of storage to iCloud.
200 (the amount of storage you bought) - 50 (amount of storage you're using) = 150gb.
That is what I mean by you purchased an extra 150gb of storage. It still adds up to 200gb, but since you were already using 50gb of it, you only get 150gb of that 200.