Extra large egg

[Shortsighted]

There were several eggs in this cup-shaped nest, but one of them was
noticeably larger. The large egg has the same colour and speckled pattern as the
smaller eggs. Is this another birds egg, or do eggs vary in size?

[Dinusaur]

Parasitism in the animal world is on display here - the large speckled egg is most likely a Cowbird egg; they don't build nest of their own, instead they lay their eggs on someone's else's nest. not sure who the smaller eggs belong to. I have seen Chipping Sparrows feeding cowbird fledgling.

[Shortsighted]

I've seen a Chipping sparrow feeding a cowbird and I realize that
they lay their eggs within another bird's nest but how is it that the
colour and pattern matches so well?

I think the nest is that of a Yellow warbler.

[Steve Hood]

That is a Cowbird's egg with the Yellow Warbler's eggs.

[cairnstone]

It's perfectly legal in Ontario (and probably encouraged) to remove the cowbird egg. From the MNR:
https://www.ontario.ca/page/remove-bird-nests-or-eggs

I'm not sure if this only applies to private property.

[GStuart]

I'm not sure about the "encouraged" part.  Beware of unintended consequences.  While the laws referred to in this paper are American and don't apply, the behavioural arguments are still valid.  Cowbird egg recommendations.