Don’t know the name of the book or anything about the subject. All I remember is a illustration of A) Either a tiny dog jumping through a ring or the hole of a skeleton key, or B)the dog was normal sized and the person holding the key or ring was a giant.
I remember the person (giant) holding the ring (key) was very amused by the dog.
The dog was with a boy who was comparable in size to the dog.
I saw this book in the late 60’s early 70’s, so the book is probably from the 50s or earlier.
Maybe Dorothy Kunhardt’s book Little PeeWee, or Now Open the Box (which I believe also goes under the name of Little PeeWee the Circus Dog or other similar variations)?
It’s an old picture book. PeeWee is a very tiny dog, and is part of a circus act, but then starts to grow to “normal” size and has to leave the circus, much to the distress of the other circus performers.
There are at least three versions with vastly different illustrations, so be aware of that when you are looking at images.