Well, the literal meaning of "He was tiny in death" is that Colin Creevey looked more tiny when dead than when he was alive. Colin was much shorter than Harry. When Colin was alive, he was always cheerful; he was filled with excitement and vigor. But his dead body had none of it. It was still and silent. The body appeared tiny.
The next sentence in the book provides evidence that Colin was so tiny and light that he could be lifted up by a single person.
‘You know what? I can manage him alone, Neville,’ said Oliver Wood, and he heaved Colin over his shoulder in a fireman’s lift and carried him into the Great Hall.
In the figurative sense, "He was tiny in death" is similar (but opposite) to "He was glorious in death."
A person who is "glorious in death" has gone out swinging. This person is a warrior; a fighter who has slayed dozens before going down. His death is not overlooked. This person has made significant contributions in furthering their fight against the enemy. This person will not be forgotten.
In contrast, Colin's death is "tiny". Harry felt that dull blow to his stomach because he realized that Colin, who was too young to be fighting in the battle, had sneaked in and died fighting for him. Perhaps in Harry's mind, Colin did not deserve to die like this—die so young, die alone, die in utter chaos.
Colin was expelled from Hogwarts as Voldemort's policy did not allow for Muggle-borns, and he was later dismissed form Dumbledore's army for being underage. He came back twice. He owed them nothing; he could have easily turned his back on Hogwarts, because the school never fought for him. But Colin remained loyal to Harry and Hogwarts until his last breathe. He was pure. He didn't have greed and hate. And despite all these, Colin's death was not as glorious as those of some others'.
There wasn't much he could have done in that battle. And it is possible that he died very quickly. It is quite likely that most in the battle did not even notice him there. And so, even in death, Colin went unnoticed. In the grand scheme of things, Colin's death is inconsequential.
This excerpt perfectly juxtaposes Colin's "tiny" death with those of others—who had rather glorious ones.
Many people in Harry Potter’s world are heroes and got their moment in the spotlight. Look at Neville, hefting the Sword of Gryffindor above his head, beheading Lord Voldemort’s beloved Nagini. Colin, quite literally, was simply content to be behind the camera. We do not know how, or by whose hand, he died – not like Dumbledore, falling from a parapet, not like Snape, laid low by Voldemort’s own hand – and there was no drama to the moment that his small body was hefted over a shoulder and laid to rest among hundreds. The ending of his little life was a small death, the kind of passing that passes by, unremembered. - Creevey’s death was saddest of all