Interesting, comparing commodity services (VMs, storage etc) like-for-like, DO has always seemed more expensive than AWS. Do you remember what was the main source of savings?
I lacked both expertise and time to find out where the wasted space go. After I've set up Maria DB on a smallest Digital Ocean droplet, mysterious storage growth haven't repeated and the cheapest droplet had enough capacity to serve our needs for years.
Also, there were 7-10 forgotten "test" server instances and other artifacts (buckets, domains, etc) on Amazon (I believe it's also quite common, especially in bigger companies).