Newbie pre-sales question

When I use the t-shirt demo, add an image that is 2600px x 2000px, 3.5mb it works fine, but when i download this as pdf or jpg the file is only 375KB. And if it is the .pdf it isnt editable.

Now whilst I dont really care about it being editable with just an image I do need the image the same size as it was uploaded. I can then cut it out in PS and then print using our Versacamm or DTG. If its is only 375KB it is only 300px x 400px. This is not large enough to print on a t-shirt

It seems odd that the image uploaded, when downloaded is only 25% of the original size

I am doing something wrong here?

If use this plugin: https://jmaplugins.com/product/tribal-skull-t-shirt-men/

all the different file formats are zipped and then downloaded and they are all editable and are the same size as uploaded

Any ideas - cheers