There are two primary reasons for a meringue to shrink. Sugar content and amount of beating both effect the volume of your meringue.
To stop your meringue from shrinking never use less than one ounce of sugar per egg white or overbeat your meringue.
Preventing Meringue From Shrinking
When discussing meringue shrinkage it’s important to understand what is going on chemically. Don’t worry though, this is written for the non chemist baker.
A soft meringue topping generally uses equal parts sugar and egg whites while a meringue that will be baked requires twice as much sugar to maintain it’s form.
The sugar to egg white ratio is critical to get right if you do not want your meringue to shrink.
Egg Whites
Egg whites are are comprised mostly of water but it also contains protein. When you whip an egg white the protein molecules are moved into new positions. Whipping an egg white causes the proteins to be less closely linked to each other and the water molecules.
This space created by whipping allows air pockets into the space between the molecules.
When you overbeat an egg white the proteins tighten together which causes the moisture to pushed out of the foam and the meringue breaks down.
The proteins will also lose their elasticity. And they will no longer stretch to allow in air pockets once baked.
Sugar’s Role
When making a meringue sugar melts into the water of the egg whites. This creates a lubrication for the air pockets. This lubrication prevents the little air pockets from merging when they bump into each other and creating large air pockets.
Smaller air pockets create a more consistent and smoother meringue foam.
Sugar also plays a very important role in preventing shrinkage by interfering with proteins linking together.
Adding more sugar allows you to beat your meringue longer without proteins squeezing out water which leads to a fuller foam.
More sugar to egg whites effectively means you can beat your meringue longer without it losing moisture.
Conclsuion
Once your meringue is baked water will evaporate while the egg white protein sets. So when making your meringue you can prevent it from shrinking by using more sugar and beating it for a shorter amount of time.