Helmet for very large head and very small heads

There are certain characteristics for people’s head, moreover, there is a general range of head circumference for different age groups. For example, S/M(54-58cm) and M/L(58-62Cm) are the most general size for adults.. But what about adults with very large or very small heads? How do they choose the right helmet for them? Would it be easy for them to get a right helmet?

Some people are with very large head. How can they find an appropriate helmet?
If helmet is smaller than your head, it would look like perching on your head, without side protection as well. Here are some options.

First you can take out the fitting pads in the top to move the helmet further down. You can also use a larger helmet with more headroom, putting thicker fitting pads on the sides. If you are lucky, you can fit in one of the helmets with additional side protection.

On the other side, there are also some people with very small head size. How these people find a suitable helmet fit them well?
Here are some suggestions as well.

For a smaller head, you can even wear a youth or children helmet that fit you well. All of these helmets pass the same impact standard, helmets for children under five even is with lower profile to provide more protection.How to make a helmet smaller?

Firstly, match it with a thicker pads inside. It not only can reduce some space but also make you feel more soft and comfortable. As well, do not detract from the protection of the helmet, as long as they hold it in place well. Secondly, adjust the rear knob to the best fit so as to keep helmet stable on your head.
Lastly, adjust the straps for maximum stability to hold the helmet level on your head while still not too tight for comfort. 
But be prepared for the helmet looking like a mushroom.