Yes, a pixel based padding makes it expected.
Thanks a lot for telling me about the subpixel rounding.
We finally decided to use 3px padding without using default .2em padding of theme.
To make our decision more reliable, I investigate the cause of behavior.
There are two causes.
For example in the last fiddle,
3.1px padding makes it unexpected.
If the fractional part of padding and border (whether it's computed or not) is lower than 5,
the computed height or width is rounded down by IE8 and it makes the element 1px smaller.
On the other hand, IE7 ignores fractional part of padding, so it looks stable.
0.5em padding that is computed to 15px in total should makes it expected.
But there is another one.
jQuery.css() computes 0.5em padding to 8px in IE < 9.
IE8 computes 0.5em padding to 7.5px when it computes offsetWidh, offsetHeight.
The difference makes resizable element 1px smaller.
On the other hand, IE7 always computes fractional em padding to integer, so it looks stable.
... I hope my English represents my thought.