Generally, pure U-235 is radioactive and decays by emitting alpha particles.

 

So pure 1gm of U-235 shows higher activity.

 

But in case of U3O8 which is ore of uranium in oxide form, contains U-atoms but these are different isotopes like U-235, U-238, U-233.

 

In U3O8 as there are various isotopes of Uranium and mainly it is U-238 which is more than 95% and is not radioactive so only small fraction of U3O8 due to U-235 shows activity.