CSS min-width property is not functioning

I have following problem:
I have div containing other elements and I try to make its width dependent from the content. I also restrict maximum width of this div. I use min-width and max-width in CSS, but it seems that min-width is not working. The width of div is always the max-width value, no matter what the content is.

Example:

I'd like to have the white div (that is main div, I was talking about) strictly around the form that is in it, without empty spaces on the left and right side. I gave the form style max-width:500px to show that even if content is small, the main div stays the same.

HTML

<div class="gInnerBox sInnerBoxMain">
    <form style="max-width:500px; margin-left: auto; margin-right: auto">            
        <div id='content'>
                   <!-- CONTENT -->
        </div>
    </form>
</div>

CSS

.gInnerBox{
    position: relative;
    background-color: #fff;
    opacity: 0.9;
    padding: 10px 10px 10px 10px;    

    box-shadow: -5px -5px 20px #000, 5px 5px 20px #000;
    -moz-box-shadow: -5px -5px 20px #000, 5px 5px 20px #000;
    -webkit-box-shadow: -5px -5px 20px #000, 5px 5px 20px #000;

    border: 1px solid #000;
    border-radius: 20px;
    -moz-border-radius: 20px;
    -webkit-border-radius: 20px;
}

.sInnerBoxMain{
    max-width: 1000px;
    min-width: 500px;
    margin-left: auto;
    margin-right: auto;
    top: 50px;
}

_{(source: wstaw.org)}