That chart is showing ONLY the PUBLIC nodes and channels. But you are missing out another important aspect: LN is having 2 types of nodes and channels: public and private (unannounced). LN is much bigger than what you see in those charts.

Here is a guide about private nodes https://darthcoin.substack.com/p/private-lightning-nodes

No, you can't know exactly how big is LN. And that is kind of good thing. More privacy.