Yes, you are using encapsulation by using properties, but there are more nuances to encapsulation than just taking control over how properties are read and written. Denying a property to be set from outside the class can be useful both for robustness and performance.
An immutable class is a class that doesn't change once it's created, so private setters (or no setters at all) is needed to protect the properties.
Private setters came into more frequent use with the property shorthand that was instroduced in C# 3. In C# 2 the setter was often just omitted, and the private data accessed directly when set.
This property:
public int Size { get; private set; }
is the same as:
private int _size;
public int Size {
get { return _size; }
private set { _size = value; }
}
except, the name of the backing variable is internally created by the compiler, so you can't access it directly.
With the shorthand property the private setter is needed to create a read-only property, as you can't access the backing variable directly.