Pngtuberplus is pretty inefficient with memory usage. That could be why. I have a high resolution png model that, when loaded, uses about 7-8GB of memory.
I pulled the github repo last night and dug into it, and there's definitely a lot of caching going on that is unnecessary. Was able to cut the memory usage significantly playing around.
(Though to be entirely fair, the software full on doesn't claim to be efficient there. The memory usage is only going to become starkly clear if you've got dozens of high resolution pngs. And if you do like what I do when making these, and export all your layers to be the same image size (so that when you import your image files you don't need to move anything), all that empty space that doesn't show up in the file size of a compressed png matters in video memory texture size.)