Text Shrinks in Fillable Forms Much Smaller Than Box Allows

I have designed a fillable form and set the "answer box" to AUTO size font, 35 Characters Limit and No to Scrolling. When I fill the form out, 30-35 characters fill the answer box nicely and text is large enough to see once printed. When some of my customer fill out the form, even just 25 characters shrink to the center of the box (instead of filling it completely) and the font size is so tiny I can't easily read it. Is there a reason this is happening? Can I fix it? I attached a picture of a normal filled box and one from a customer that "shrunk".