Latest ACR update - possible bug while using brush tool and moving around image?

I recently updated Photoshop and ACR and have found a potential bug / annoying issue. 

I use a mac book pro and zoom into the image using the + or - keys. When creating a mask and zoomed in very deep 100% I used to be able to swipe with my track pad left and right, up and down to move around the image. Now when I do this, the brush size becomes smaller or larger!!!! 

The issue goes away if I hold the space bar down, but its a bit of a hassle as I am doing this method frequently. 

Does anyone know how to change this back to the origibal settings? 

Thanks, 

Matt