Well this happened to me multiple times, first I thought it's an issue with my BBCode, but I guess there is some issue with forum css.
This happens when you add a table with an image in it. For example, I added the below table with an image in the first column.
I did not specify height or width but still its shrinking
Now even if you try to put e.g. height=800 and width=800, still it will shrink.
It's a small issue but still maybe theymos you need to check.
I have noticed it's coming from below CSS of img.userimg
style.css:649
img.userimg {
max-width: 100%;
}
or either this
style.css:555
div#bodyarea table.bordercolor td td td.td_headerandpost td:first-child {
width: 26px;
}