The short answer
BR40 is wider and usually brighter. It may not fit a shallow or narrow recessed can designed around BR30.
SpecificationBR30BR40
Nominal diameter3.75 in5 in
Typical can5–6 in6 in
Light outputModerateUsually higher
Common baseE26E26
Before you substitute one for the other
Matching the E26 base does not guarantee the larger BR40 envelope will physically fit.