The smallest mammal (Etruscan shrew) is about the same size as the largest ant (Driver ant Queen). You probably need to speed up the tiny-lion metabolism, but in general the mammal body plan can work around that size.

On the other hand, to scale up insects to the size of a lion, you would need to completly redesign the respiratory at least and the entire exoskeleton construct will also be pushed to it’s structural limits.

Downscaling mammals is much more realistic then upscaling arthropods.