This is the best I could do with my phone's camera.
Grado size S on the left, Sennheiser size XS on the right.
Comparison between Grado size S, Sennheiser size S and XS.
Dimensions are similar as you can tell. Sennheiser S is a little taller than Grado S. I use the Sennheiser XS on my Grado's.
I'm not sure why they sound different as I did have a good fit with the Grado ear tips. I suspect maybe the silicone is different? The Grados do feel a little flimsy compared to the Senns which are a little more dense.
I highly recommend the Sennheiser tips though if you're not happy with the sound. They can be had for $11 directly from Sennheiser.
***I tried the stock ones again just now just for kicks and I immediately could hear the droney overpowering bass.