Much more or much better than you thought was possible.

这是一个主句省略了成分的比较结构。

much more or much better，可能是主句的表语。还有一种可能，much more 是宾语，much better 是状语。

补充如下：

1. It was much more or much better than you thought was possible.
2. I did much more or much better than you thought was possible.
1. 这比你想象的要多得多，或者好得多。
2. 我做得比你想象的多得多，或者好得多。

比较从句 than you thought was possible 中，you thought 是嵌入成分，类似还有 you imagined, you believed, you said, you expected 等。去掉就好懂了：

...than was possible.

从句中，暗示 than 是 was possible 的主语。一些类似表达：

He never says more than is necessary.

他从不多说没有必要的话。

He often eats more than is good for his health.

他吃的饭量往往多于对身体有益的量。