From Harry Potter and the Sorcerer's Stone, Chapter 4:
From an inside pocket of his black overcoat he pulled a slightly squashed box. Harry opened it with trembling fingers. Inside was a large, sticky chocolate cake with Happy Birthday Harry written on it in green icing.
A few pages later:
The giant sat back down on the sofa, which sagged under his weight, and began taking all sorts of things out of the pockets of his coat: a copper kettle, a squashy package of sausages, a poker, a teapot, several chipped mugs, and a bottle of some amber liquid that he took a swig from before starting to make tea.
Also in the same chapter:
...and from yet another pocket inside his overcoat he pulled an owl — a real, live, rather ruffled-looking owl — a long quill, and a roll of parchment.
We see that Hagrid is able to carry around so many things in his coat. Do we know if it's just an indication of how large his coat (and pockets) are, or might there be an extension charm on the coat?