Those are antoninianus dies, as shown by their size and by Philip's radiate crown on the obverse. So it is an underweight antoninianus. A denarius would have a laureate portrait, and be struck from smaller dies.
True, I should have seen that from the start( the
radiate crown).
Antoniniani are always
radiate. Small surprise it is underweight, many
Antoniniani are, that is why I despise the term "
double denarius", which would seem to indicate a coin of at least 5 grams, up to 6. I love looking at coinage and how the
quality of it fluctuates along with the state of the Empire. Chuy1530, to your question, it would not be out of the ordinary for coins like yours, struck with debased silver, to also be made "light" on purpose, or out of necessity as silver
supplies dwindled.