What exactly representational bottleneck in InceptionV3 means?
Cool question! I managed to start thinking about its meaning thanks to your asking.

I believe you would like to consider it in the sense of information flow. As the information is compressed to a same size in both cases, the question is which one is more effective. Information with amount of 35*35*640 can flow through a wider pipe more freely. A pooling layer just prunes information directly in the left solution, while doing Inception in the right case expands the representation to a more sparse space and makes meaningful inferences before get pruned. Think of the pooling operation in left as a neck or a narrow path, and the middle representation in right as a broad lake. The pooling layer in right can also select better output since the source has more abstract (semantics) information.

As to figure 10, I consider it as a balance between the representation size and the computation burden.