You really should be more detailed about exactly what you are looking for. There are TONS of weapons that meet the requirements you are looking for.
How small and light are you willing to go?
Does ammo capacity matter?
What is the purpose for the smaller weapon, normal CCW gun or just a summer carry piece?
Do you want something more like your G19 (Doublestack, Striker fired) or do you want something else DA only, DA/SA, Manual Safety, Single stack (thin)?
You also have to remember that there will be compromises you'll be taking the smaller you go.
The effictiveness of ammo is reduced due to short barrel.
Because of lighter weight follow up shots will be slower due to more "snappy" recoil.
Smaller sights (or non existant ones on some models) and shorter sight radius will make you less accurate then you are with your G19.
Shot placement... Can you become accurate with the smaller weapon? Less power makes this even more important.
Less ammo means you'll need to carry reloads especially since your ammo effictivness has gone down (might take whole mag to stop threat).
I'd recomend you go to the next step down to a Glock 26 from your G19 if all that matters is smaller and lighter. It will be similar to what you already have but smaller and you can use your G19 mags in it. It will be a good compromise for you and you won't loose much in capacity or accuracy from it. You can usually find some used ones or police trade in's for cheap too!