It is because the x-t2 files are compressed. Importing them into LR doesn't change the size of the file.
Lossless compression is, by definition, the same quality as the fullsized raw. Think of it like a zip file. You can zip the files up and they get smaller but when you unzip them they are the same as before you zipped them up.
The cards may be cheap but long term storage of many thousands of images at 50% larger adds up quick.