Putting drop bars on a bike with flat bars isn't a simple thing. If you didn't change the stem length then your reach is changed from day one.
I've been of the opinion that a bike made specifically with flat bars in mind has a slightly different frame reach than a similar road bike with drop bars. So a flat bar bike that fits a person well might be too big for that person if drops are put on it.
And with the different hand position, width might be a factor too. Narrow is better for me on drop bars.