The map will shrink, just not by as much as you might expect. You could also argue that they have picked a pathological case for their example because they use

The map will shrink, just not by as much as you might expect. You could also argue that they have picked a pathological case for their example because they use 128 byte entries ("If a key or a value is over 128 bytes, Go won’t store it directly in the map bucket. Instead, Go stores a pointer to reference the key or the value" - which probably leads to less memory consumption).