Side by side the gz looks to be a bigger bike that is b/c the gas tank is noticeably larger/wider. When riding though the s40 allows me to sit back at a more comfortable position for the legs - I am 5'7" and 160lbs. They both are comfortable for my arms neither reach or scrunch.
It took me some time to mentally adjust to the maneuvering capabilities because I ride both bikes daily. You can get away with things on the gz that can cause you problems on the savage. The 650 is heavier (obviously) but where the weight is centrally/forward and higher up makes slow turns (parking lot) and tight traffic more challenging but not difficult - I guess just different.
ps I find the seat on the gz more comfortable and many savage/s40 owners replace the seat if they take longer rides. Some harley sportster seats have an easy mod as a replacement.