A:Answer Hi Julie, The only difference between these two are the weights and measurements. The XB30 dimensions approximately 228 × 82 × 86 mm, weight is 980 g and for XB40 Dimensions approximately 279 × 100 × 105 mm, weight is 1,500 g. You can check the specification of these models from the link below:
https://docs.sony.com/release//SPECS/SRSXB30BLK_mksp.pdf
https://docs.sony.com/release//SPECS/SRSXB40BLK_mksp.pdf
Regards, -Jay