Tensorflow VarLenFeature vs FixedLenFeature

I was trying to save images of different sizes into tf-records. I found that even though the images have different sizes, I can still load them with FixedLenFeature.

By checking the docs on FixedLenFeature and VarLenFeature, I found the difference seems to be that VarLenFeauture returns a sparse tensor.

Could anyone illustrate some situations one should use FixedLenFeature or VarLenFeature?