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Whales are users who hold a lot of HP (not sure how many, i guess over 200,000 - i'll try to find out). They therefore have large curation votes, witness votes and DHF proposal votes, so it's difficult to earn decent amounts on posts/comments without whale votes, difficult to get a DHF proposal funded or get into top 20 witnesses without whale support. It can therefore be hard to make changes which do not financially favour whales in short term. However there are some whales who play the long game so will support.

Hope that clarifies.