nested size tags get ignored

In converting some posts from phorum->drupal I found some combinations of tags which won't work:

Here is an example.

[size=15][size=10]other[/size] [size=20]super[/size]nested[/size]

This outputs: [size=10]other supernested[/size] where other is 15, super is 20, and nested is 15.

I expected that other woulud be 10, super would be 20, and nested would be 15. I also expect not to see the [size=10] and the [/size] but it is visible.

Another combination I tried was
[size=15]prefix [size=10]other[/size] [size=20]super[/size]nested[/size]

The results were similar:
prefix [size=10]other supernested[/size]

It's not a huge problem, but I didn't find it in the queue so I wanted to make sure it was reported.