Magento – How to change the width of main column in 2 column layout in Magento 2

You can adjust the width in the stylesheets:-

.col-main {
    float: left;
    width: 75%;
}

.col-right {
    float: right;
    width: 25%;
    padding-right: 0;
}

Just adjust the percentages to the width you want them to occupy.

Alternatively, rather than modifying the width of the columns in the 2-column-left, 2-column-right or 3-columns layouts, you could create your own custom layout...

Lets debug, in view.phtml please add below code for getting template

$this->getLayout()->getBlock('root')->getTemplateFile();

If it given value ...page/2columns-left.phtml then the template file properly applied at your theme.Then at any where force full set template for product details page. and you need to check all layout files,.

Also it give proper result then you can to set 2columns-left.phtml to all pages by using handler as default <default></default>.

<default> <!-- this handler is called  on all pages -->
    <reference name="root">
        <action method="setTemplate"><template>page/2columns-left.phtml</template></action>
    </reference>
</default>