Select Tool (to select area for noise reduction) not working on larger images (PS CC)

Hi all,

I've searched but have not found anything that resembles my issue.  

I am having a problem with the select tool when I want to select a specific area to reduce noise, etc.    The weird thing is that the select tool works just fine on the same image when I resize it to something like 1600 pix wide, and it 'intelligently' selects the area.   When trying to do it on the full size version of the same image, the select tool is acting like a drawing tool and does not let me block off areas that I want to work on specifically.    Even if I use the select tool like a 'smart lasso' and try to connect the 'dots' to complete a selected area, only the line that the tool draws is affected by any changes that I try to make, like brighten, darken, etc.

It baffles me why the smart select tool works in 2 completely different ways on the same image when the only difference is the size of the image.

Can someone point me in the right direction?

Thanks!