Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear about your little one's consistent regurgitation of water, slow eating, and small size compared to her parents.
I agree that there is likely something going on with your girl, even if externally she seems to look fine. She should not passively reflux water every time she drinks, and her small stature likely points to some sort of health problem that interfered with her growth
. In a young dog that can be poor organ development and function or since she seems to have trouble with regurgitation perhaps a problem with esophageal function.
Dogs with poor esophageal function may be able to swallow food, as gravity helps pull the food into the stomach
, whereas liquids more easily splash back up.
Has she had any blood work checked to evaluate organ function or radiographs taken of her neck and chest to look at her esophagus?
I would start with a biochemistry profile and urinalysis to look at organ function and radiographs of her chest and abdomen. A barium swallow, which is a series of radiographs taken after giving her some liquid barium to drink, can outline her esophagus and see how it functions with liquids.
In the meantime you might try having her in an upright position for several minutes after drinking. Put her front feet up on a chair or stool after drinking for 10 minutes or so to allow gravity to help pull water down into her stomach and keep it there.
Please let me know if you have any further questions.