You can buy quite a few sizes with reduced shank but beware that chatter is more likely.

Another option which is meant for the use you describe are the reduced neck milling cutters which have an area above the shank reduced in diameter but are full diameter on the plain shank. I’ve not had chatter issues with these. I’ll take a photo when I’m in the workshop later

Insert shell mills will also allow similar milling operations

One word of caution if using a 14mm with 12m shank is that you should where ever possible have the minimum of stickout so if possible put the 14mm close to the end of the collet. A long series cutter will allow you to reach further while still leaving sufficient shank in the collet.

So here we have a cutter with a smaller shank in the middle, in this case a 1/2″ 4-flute on a 3/8″ shank.

The two on the right are commercial “necked” cutters, a 2 and a 4-flute where the flute only extends to where I have drawn the arrows and the rest of the coated area above is approx 90% of cutter dia so the 10mm cutter reduces to 9mm but has a 10mm shank and still has plenty to hold.

On the left are two long series cutters the main thing to noe here is that there is enough shank to completely fill a collet but when the reduced shank one is extended to a similar length you don’t have  alot to get hold of so starts to get a bit risky.

If you really want to pull a cutter with a small shank and larger flute diameter out of the collet to extend the cut then use the proper ones that have extended shanks so you maintain full grip, I don’t have any but this is the type of thing.

PS also check the spec before you order that mus have 14mm cutter as some have 16mm shanks and some have 14mm shanks not all are 12mm shank.

I’ve made a such things from silver steel by turning and filing. They are far less elegant, not perfect, but work.

Neil