Lightroom classic adding extreme amounts of grain, mobile is not

LrC does not add grain or noise to an image, unless you deliberately add it yourself.

You have several tools for reducing noise in the Detail panel – Denoise for very noisy images, and manual noise reduction for more moderate noise.

When working with noise, sharpening, or other image detail, you have to view the image at 100%.

At this magnification, one image pixel is represented by one screen pixel, and this is the only view that presents you with a true representation of the image. Any other magnification will be inaccurate and misleading because the image has been scaled. To see what the image really looks like, view at 100%.

The 100% rules applies to all situations, also exported files. It also applies to Lightroom Mobile, which I'm not familiar with.

When sharpening a noisy image, make sure to use the Masking slider, which protects flat areas, where noise will be most prominent, from sharpening. Press the Option key (Alt key on Windows) while dragging it to see the effect. White areas will be sharpened, black areas will be protected. A high value is needed for noisy images, typically in the 50 to 90 range.

Which ISO do you used in the camera? 

Which export settings do you user in LR Classic?

... I want the pics to have a larger file size than they would if they exported from mobile, but literally everyone comes out completely ruined like this example. 

...

By @henry_4597

What do you mean with "larger file size"?  What do you want to smell with it?