The CMS shouldn’t downsample images by default… it’s probably being told to do so in the template code. So, the first place to look would be in the template files (with a .ss extension) in the themes directory. You might find that they have image manipulation functions such as
$Image.ScaleWidth(500) or similar. It will produce thumbnails for use in the CMS system.
Silverstripe is generally pretty good at not being too opinionated in the way it works, so it’s not usually necessary to try and circumvent it.
Trying to edit content directly in the database, or by overwriting files is going to ultimately end up being very time-consuming and frustrating, since there’s a lot going on behind the scenes such as versioning, etc. which is masked by the CMS.