What is the difference between .ecdata and .ecube data files?

The Elasticube Manager stores the design of the data schema in a .ecube file. It is not the data itself, but actually a “blueprint” of your data structure and source definitions. The .ecube is the data itself already in the Elasticube. You can notice that .ecube files are very small (a few KBs), whereas .ecube files are sensitively larger (may reach GBs  depending on your data).