Hi,
To be fair you cannot say for certain that this is purely a care issue without more information than we currently have.
There could be structural or developmental reasons the snake couldn't eat what we think of as a normal sized meal for its age. In the pic the narrowness of the neck realy looks strange even allowing for the extreme swelling of the head and throat area.
I certainly don't think the OP was intentionally underfeeding their snake but was more likely to have been getting bad advice from somewhere even if the issue turns out to be care related.
I just don't want this to turn into a bashing session. :(
dr del