Estimates should always be done by the people who will be doing the work. You should only estimate if you are a developer in the team that will be doing the work. This applies to Epics, Features, User Stories, or any other type of product backlog item. The “Epic-level” items should be decomposed into smaller chunks that each team can estimate on their own. The estimates from each team can then be added to arrive at a large-scale estimate of the Epic. In Scrum@Scale, this process creates a two-way street where items are decomposed by the Chief Product Owners, while estimates are provided by the people who will be doing the work.