used to express sympathy over a petty matter - British informal
Related Idioms :
a big cheese
an important and influential person – informal
These are mainly self-explanatory with the exception of cheese itself, which is of doubtful origin but may be from Persian and Urdu chTz meaning thing. As a phrase, big cheese seems to have originated in early 20th-century US slang, as did big noise. Big wheel in this metaphorical sense (as opposed to the fairground ride known as a Ferris wheel) and big shot are similarly US in origin (mid 20th century). Big fish may have connotations either of something it is desirable for you to catch or of the metaphorical expression a big fish in a small pond.
said by a photographer to encourage the subject to smile