Why is the file size of photo from my XS only 1.2 MB while a photo from my wife's 6S is 3.8 MB?

I'm with Jonathan Burger as to the primary reason. On newer devices Apple changed the format used to store photos and videos to a more efficient codec. It can be problematic because is it so new that not many applications can use it so if you need to you can change the settings to use old codecs.

"I have checked High Efficiency"

There is your answer.

"The MPEG group claims that twice as much information can be stored in a HEIF image as in a JPEG image of the same size, resulting in a better quality image.' -wikipedia

Was the XS photo taken in low light where the 6 photo was bright light?

Take two identically lit photos to compare. ( maybe they were but you didnt say)

What resolution have you set the camera too?

https://developer.apple.com/library/archive/documentation/DeviceInformation/Reference/iOSDeviceCompatibility/Cameras/Cameras.html

The Xs has 12 meg to 8 meg on 6s camera. So that’s why you have a different in image size.

There are many possibilities. Photos are stored as JPG or HEIF (it's a setting in Settings/Camera) Both are compressed formats. HEIF will be slightly smaller than JPG. But unless the photos are of identical scenes the sizes will vary with the number of colors and color changes. Both formats use standard compression algorithms that replace repeated pixels with a single value and a count of the number. So, for example, if you took a picture of a single color sheet of paper the image would be very small, but a photo of a Jackson Pollack would be huge. Also consider whether the image is a Live Photo (which is essentially a rapid sequence of still photos - a minimovie) or not.