One reason I moved away from DSLR's was the increasing bulk and weight of the system which includes lenses. No way would I take the behemoth existing lenses, add an additional adapter for more weight and bulk and then stick that only a mirrorless body
It is roughly a wash for total camera bulk, lens plus body. The extra bulk of the adaptor simply replaces the extra bulk at the front of an F-mount or EF-mount body due to the SLR lens mount needing to be further from the sensor (for example, 44mm for EF vs 18mm for EF-M). A mirrorless camera with SLR mount must have a bunch of empty space where the SLR mirror assembly used to be: look at how bulky the Pentax K-01 (mirrorless with SLR mount) is compared to Sony E-mount bodies in the same format—and the K-01 does not even have an EVF adding any bulk!.
So when reusing SLR lenses on a mirrorless body, it is a wash, and likewise when reusing SLR lens designs in mirrorless mount: camera with lens is the same length, but with longer lenses and a shallower body. However, when using lenses designed to take advantage of the shorter registration distance, the new mirrorless mount often wins on total camera bulk. For example, compare the Olympus 12-60/2.8-4 lens for Four Thirds SLR mount (registration distance 38.67mm) to the Panasonic Leica 12-60/2.8-4 lens for Micro Four Thirds mirrorless mount (registration distance 19.25mm):
FT lens: W 79.5 mm, L 98.5 mm, 575 g
MFT lens: W 68.4 mm, L 86 mm, 320 g
(And that MFT lens has in-lens OIS adding some bulk, which the FT lens does not.)
Adding in the registration distance, the FT lens extends 137.17mm in front the sensor, while the MFT lens extends only 105.25mmL a difference of 32mm or 1 1/4 inches.
This is an example of how optimal mirrorless system design (not overly constrained by backward compatibility) allows, in your words, for "a more lighter compact camera and lens system."