What do you mean by cosine losses? I have done research in fibre optics for more than a decade and never heard that term. Also I have no idea what you mean by light bends around the fibre. A simple explanation of light propagation in multi-mode fibres can use geometric optics to explain how light propagates along the fibre by reflecting of the interface between core and cladding, however this simple picture does not apply to single-mode fibre (which all long distance connections are) and also does not easily explain the group velocities in fibre.

The reason that light travels slower in fibre is because the refractive index of slica is about 1.5 while it is 1 in glass (in reality it's a bit more complicated, it's the group index that counts, which is also approx. 1.5 however).