At the moment, VisualEditor asks for details from the API on each edited image to get its original dimensions. When this is done for the edit dialog the asynchronous call works fine because we don't mind letting the user wait the seconds it takes the async call to finish.
But when the user resizes images with the mouse (with the resize handles) the information about original dimensions usually comes too late. The user may (and often does) resize the image to above its limits, and by the time VE gets the API response, the user is already done resizing. On top of that, calculating aspect ratio from original size is a lot more accurate than calculating it from the given (usually smaller) size, though, granted, this is a lot less crucial.
We have some fall backs and we are considering an after-the-fact user experience ideas to make this work, but it would be extremely helpful if the information was already available when it comes from Parsoid -- especially since Parsoid already asks for information from the API.