Both this post and the source screenshot say 2 kilos, but Musashi's breasts look a few times larger.
To map breast weight to approximate bra size, you can use the table on Wikipedia here. The closest match is the following US bra sizes at 2.1 kg (4.6 lb): 44C 42D 40E 38F 36G 34H 32I 30J 28K.
It doesn't look like the girl in the source picture has breasts quite that large?
To estimate Musashi's bra size, a close real-world counterpart is Lucie Wilde, who has a similar build and is just 1cm taller. Lucie is 34L and Musashi here looks at least as big as Lucie Wilde. 34L = 3.7 kg (8.2 lb) 34M = 4.2 kg (9.3 lb) 34N = 4.8 kg (11 lb) 34O = 5.3 kg (12 lb)
My breasts weigh togetherWhat is something you like to boast about yourself?Servant observation. Beach edition in Chaldea.About 2 kilos