The lighter weight of a Grom would make it more forgiving until you get onto balance, use of clutch, use of brake. Get to leaning wrong on a heavier bike and it can drop. You might be able to save the Grom at only 225 lbs if it starts to fall over.
We do not have classes of bikes in the US. The only distinction is between a moped which most states limit to 50cc and about 30 mph which requires no motorcycle license (with some exceptions) and anything larger which does require the motorcycle license to be legal. In Texas, a Metropolitan and Ruckus both with 50 cc. are classified as mopeds, but the 50cc Aprilia Sportcity is considered a motorcycle because it goes a bit faster...42 mph or so. Both mopeds and motorcycles require a license plate that generally costs the same. Real moped owners object to the use of the term moped to describe anything without pedals, but the states use the term to define what is and is not a legal 50cc or under motorized vehicle. Some of the small scooters have what are called restrictions to top speed...things like smaller exhausts in order to slow the 50cc bike down to legal speed limits (ususally 30 mph in most states), but that is a performance restriction to allow the bike to be called a moped. The first thing everyone does, is circumvent those mechanical restrictions to get a few more mph. If you have a 100 cc. bike it will be called a motorcycle even if it only goes 30 mph....50 cc is the cutoff in all states far as I know. Many people call any small bike a moped, but not the states who apply registration laws. Oh, a moped requires a regular car driver's license though, or a special moped license, just not the motorcycle license in addition.