Because the image is cropped by 800px width. Please try to edit the file:
../template/single/portfolio-template.php, find line 31:
<img alt="<?php echo get_the_title(intval($portfolio[$index])); ?>" src="<?php echo esc_url($thumb[0]); ?>" width="800" class="list-layout-img gallery-images-img">
update to:
<img alt="<?php echo get_the_title(intval($portfolio[$index])); ?>" src="<?php echo esc_url($thumb_full[0]); ?>" class="list-layout-img gallery-images-img">
Also, Why can’t I use read more tag in portfolio? The read more tag does not appear in portfolio.
Did you talk about Portfolio module? There is not the “more” button in origin design.
.
WordPress Customization Service from 
