Is actual size of jpg (right click properties) of windows viewer correct?

The file size you are seeing in properties is the
correct current file size. A mouseover should display
a tooltip with the same info.

JPG files are highly compressed...when you open
them in an editing program they are decompressed.
That is the reason for the change in file size. When
resaved the file size may change again depending
on the compression algorithm utilized by the editing
software.

Every time a .jpg is edited and resaved it loses
some resolution. This can be noticable in a very
low resolution photo...640x480ppi for example. 
In the very high resolution photos we have today
from 8 - 12 - 15 megapixel digital cameras...the
loss is negligible and would not be noticible to
the naked eye.

If you had very important photos and wanted to
insure that no loss occurred you could save them
in a lossless format like PNG or TIFF but the file
sizes would equal that of your uncompressed .jpg.

This could be very important in a situation where
the photos would be printed at very large sizes.
High resolution images can be a real asset when
a small portion of the image needs to be cropped
as a separate photo.

An important factor for commercially printing photos...
books, magazines...the editors usually request photos
that are at least 300dpi.