300k doesn't sound very high, but it depends on what is in your illustration.
If you capture the illustration to the photos library or creative cloud , it's being saved out as a compressed .png
Currently, there is no setting to change the capture size on iPad.
If you need a JPG specifically, I would recommend sending it to Photoshop so you can define the custom export profile.
I would highly recommend sending it to Adobe Illustrator, then using the vector image to scale to any size you want. You can export from Illustrator as a JPG as well.