Real indicator for more decentralization should be growing number of full Bitcoin nodes, but I don't think we can have real accurate data for that.
Some say there are around 100,000 nodes, than there is Luke's script showing there are 75260 nodes at the moment, and bitnodes website is showing around 10380 nodes.

I don't think blockchain size should be such a big issue anymore as disc prices are going down and capacity is going up very fast, so we can expect more people will run full nodes and only this can make Bitcoin more decentralized over time.
Other issue is that I think most people don't even understand importance of running a full node so they never even tried running one, but we surely can't expect newbies running full nodes.