This is a cryptic definition - the clue uses wordplay to describe a well-known phrase or idiom that means the same as the definition.
Think about what 'big Brits' could refer to - not their nationality, but their physical description. This connects to an idiom about prosperity and abundance that literally describes the best parts of something.
Answer: THE FAT OF THE LAND | This is an idiom meaning 'the best in everything' or abundance. 'Big Brits' suggests fat people, and 'the fat of the land' (or Britain) is a biblical phrase describing prosperity and the best of what's available.