Translation:The wolf is drinking a big glass of milk.
It's Storbritannia. Stor can mean 'big' or 'great'. You'll find that 'great' is actually a synonym of 'big' in English.
'great' can also refer to being the geographically wider consideration of something. 'the Sydney area' vs 'the greater Sydney area' (which includes more than just the middle)
I am not sure of which explanation is etomologically correct, but those are two reasons i can think of.