It's one of my favourite German sayings (German native speaker), and I also haven't been able to find a good translation for this proverb.
As for the meaning, it's sort of an amalgamation of: "Don't put the cart before the horse" and "Things are never as bad as they seem".
My mom always told me "There will always be someone smarter than you," when she was trying to curb my big-fish-small-pond effect. It didn't fully hit until I went to a different school. After I went through that transition, I would say that "I had gone from being the cream of the crop to being the bottom of the barrel".
There is chinese saying, waiting for the hare by the stump/tree, where a villiager was sitting by a stump, a hare run out of a bush, ran into the stump and knocked it self out. The villager started to wait everyday by the stump for another hare.
A rude surprise. The fastest camel in Giza tried out for the races in Cairo and got a rude surprise. When BlackBerry saw what the iPhone could do, it was a rude surprise to them. It's most commonly used in future tense: if that camel goes to Cairo, it will be in for a rude surprise.
The first usage I found was an article "A rude surprise could be awaiting ...
There is a question on Quora asking for the English translation of this from Hindi.
It suggests a couple of other idiomatic English versions, such as:
someone has been cut down to size
some one has been shown his place
I'd like to add (for reference, for future visitors) a couple that were more common, at least when I was learning to read:
The following phrase is something that approaches closely the translation that is sought. It implies the possible understanding of an equality but superiority is taken into account as well (ref.).
He had met his match. (She had met her match.,…)
The Free Dictionary
There’s always a bigger fish conveys a similar concept:
No matter how large or intimidating a person or thing is, there is likely to be an even larger or more intimidating person or thing somewhere.