I find that using those lossy png optimizers like ImageOptim (MacOS) can get file sizes around the same or even lower than use jpg with masks.
Definitely a good option as well. I ran the same 1.9mb PNG through ImageOptim and it got it down to around 600kb so still about double the size. Certainly a great improvement though, especially if you have a batch of PNGs to do, but you do compromise some quality with lossy compression.
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