Most or almost all IEM tips are curved. Nozzle tip being the smallest, and it gets wider until they are widest at mid point, and some 3/4 of the tip.
 
Is there a correct/optimum depth IEM should be inserted in the ears? should you insert with just the 1/4 of nozzle tip, 1/2, 3/4, or the whole tip into the ear.
 
Thank you for the help.

There is no wrong or right answer for this, unless you're not getting enough bass from a shallow seal, or red liquid is coming out of your ears; since everyone has different size and shape ears and ear canals.
 
As long as you're getting a good seal (bass is present), it's comfortable with no pain and the tips don't get left behind in your ear canals when taking them out, it's all good.
 
I've observed people wearing IEMs in public, and sometimes it looks like they're about to fall out, and other times it looks like the buds are getting swallowed whole.

Thank you GREQ for the reply, I was very curious as somehow seal of eartip doesn't maintain for very long for me, especially when I'm moving and on the bike, always have to adjust it. When I try to insert deeper it touches the wall in the canal and restrict the sound. Wonder if there is a way to solve it.

Tip rolling.
Just try out different size and material tips.
 
Spare tips are cheap pretty much anywhere and can be bought in sets of various shapes and styles. 
Maybe you need a flanged tip, or a memory foam tip.