large heart normal?

Hi, our cat had a chest x-ray three months ago that showed everything normal
but the vet said her heart looked mildly enlarged. He advised just rechecking
in three months, since she had had iv fluids that day during a dental cleaning.
We just had her recheck x-ray and he said her heart looks the same. He said
there is no sign of heart disease and since she acts normally (active, no
panting after exercise, eating normally, etc.) he said he thinks that although
her heart is larger than normal, it could just be normal for her. He said the
size of the heart is within the range of normal sizes and is not abnormally
shaped.

I asked about an ultrasound to be sure, and he said he doesn't think it's
necessary but will arrange for one if we want the reassurance. I think we
probably will have the ultrasound, but my question at this point is whether
anyone else has had a cat whose heart was large but it didn't turn out to be
any sort of health problem? Our cat, btw, is six years old, normal weight, and
has always been more active than our other cats.