The difference in strength between a hollow axle and solid axle in minimal. Strength and stiffness is proportional to the diameter to the 4th power. Let's say you have a 10mm diameter axle that will break in bending at 10000 force units; if you drill a 5mm diameter hole down the center it will break in bending at 9375 force units - a difference of 6¼%. To totally recover from the 5mm hole in the axle, the 10mm would have to change to 10.153mm diameter. This is basic Engineering.