Sometimes the miniature accessories we purchase are perfectly scaled at one inch=one foot but they can still look like they are not correct. That is the case with these Houseworks windows I bought. I spent a lot of time painting and aging them but something was wrong. Then our Chair Ron Hoess reminded me that windows in urban structures were set into the walls. The trim around these windows is too wide to do that (these are made for wooden structures like suburban houses). So I cut them down simply by using a sharp blade on the backside. I think they will be more to scale. Look carefully at all your miniature components. Just because they are technically to scale doesn’t mean they will look right.
Another example of this is the mailbox I had last year in The Birds. I got the measurements for it from the US Postal Service website so it is the correct size but to the judges it looked too large.
This is The Birds as it appeared in Singapore, with a bird sitting on the mailbox. Perhaps the bird makes it look more in proportion.
Flower Show Miniature Settings 
I believe I might be able to offer some insight. The house is obviously vintage all the cues are there, and skillfully done too I might add.With all the accompanying prior images EVER seen by these judges of this sort of scene their eye is trained for those proportions. You must remember however that contemporary to this scene packages were not common; you received a slip to call for your package or make arrangements for someone to be there to sign for it. Hence the mailboxes were narrower in those times. Though your measurements were sourced correctly for now they would not have been that large in the period of that scene so it just looks “off” a little.
Yes but check out this image of Annie at her mailbox. It is quite large and looks to me like boxes today!
https://thewonderofminiatures.com/2013/06/23/the-garden-annie-deserves/
https://thewonderofminiatures.com/2014/02/18/annies-mailbox/
This is all true and validated by the fact of photography; however, the part was played by Suzanne Pleshette who, according to wiki, was 5 feet 4 inches. Using her as a reference point is going to be misleading to you. I have a friend that still has an old school one that I will measure for you if you would like.