To explain why we can define God to affirm his existence!

[Esquilax]

Either greatness is derived from imagination or it's derived from real existence.
A thing that points that it's from real living existence is that our good actions, the more "life" force they have, the more praiseworthy they are.
Furthermore, if we simply imagine, but don't actually bring to existence our actions, it's obvious existing actions are more praiseworthy and great.
This shows greatness is derived from existence, not mere imagination.

Do you agree with this much so far?

You aren't talking about greatness, though. You're talking about human opportunities to express how great a given concept is. Obviously if you do a thing instead of keeping it to yourself then more people will be able to praise it, but that's not an increase to the qualitative greatness of the concept, just an increase to the amount of people able to express that it is great.

The problem is that you mistakenly think that the distinction here is between the greatness of an imagined thing versus the greatness of that same thing enacted for real, when in actuality it's a distinction between how much humans subjectively value those two states.