Deep Armpits from Breast Implants? (photo)

Interesting, I have always thought about that but never seen it. I assume your muscle is tented and because you are so thin it exaceberates your arm pit. I would continue with exercises to stretch your pec muscle which should soften this

At this early stage the pectoral muscle is tented up, and exaggerated by implants that are usually high riding, with the fluid contents bulging some in the upper poles of the breasts. As the muscles begin to relax to accommodate the implants, this will resolve and the axillary anatomy will be close to it original state. All the best. 

At 6 days postop you are just over the hump for maximum swelling.  Yes, your muscles are tented up at this stage and will gradually settle down.  When you tissues swell they also get firmer with less give, so what you see is a combination of these 2 effects on the muscles.  That front fold is the tail of the pectoralis muscle, which your implants are under when doing submuscular.