It is an old saying. "Too big for your boots" or "Too big for your britches/breeches" [often said to children, the implication being that the child is insisting that they be treated as older (i.e. bigger) than they are in reality (their real age being measured by the actual size of their clothing)].
Where did the idiom too big for your britches originate and what was the original meaning?
As for the origin there are a number of claims because there are other sayings with the same 'meaning'. Your 'britches' is more used in America and here in GB it it 'boots'.
It appears to have been around for as long as we have had boots and so a bit of deep digging is suggested, using the internet of course. When you do this you will find that your sources will all quote much the same and the original meaning is what we use it for today.