It’s the size of the first Birkin, made by requirement of Jane Birkin. The Birkin 40 is particularly popular for traveling and is especially popular with men. However, the largest model of the Birkin Bags is relatively hard to get hold of and has a relatively low demand. This could be mainly due to the relatively high weight of the model and the structure of the bag - if there is little content in it, the bag tends to fall in a little and thus looks less noble.