Does the weight of iOS devices change based on storage size? I don't think this is just true of Apple products but the weight advertised for iOS device model of varying storage size is always same.

For example the weight of the iPad Pro is 437 grams. Is this the same for all models?

Does an increase in storage size add extra weight?

Is the weight difference so minuscule that it is not mentioned?
The 128GB and 256GB variants don't have any more chips than the 32GB model. They still have one chip, that has smaller cells and hence higher storage density, but it takes up the same volume and weight.

Even if this is not true, the difference in mass is likely to be within a gram or two. It's not significant enough to note.

Source: iFixit teardown for the iPad Pro. As you can see, the logic board has no space for any additional flash memory chips.