all you need to measure it is one of those soft clothing tape measures. You're boobs look perfectly fine. the unfortunate truth is as a trans person its sometimes hard to see the progress that everyone else does. breasts can also be a sore point for some because a wider torso will cause the breast fat to spread more along the chest which gives the appearance of a small breast. nevertheless you do look completely normal. i have included a link (nsfw) of a database to shows examples of normal cis-breasts of all sizes. you'll note there are cis-women out there with larger boobs and there are in fact cis-women out there with smaller boobs. you're boob size is determined by your genetic lottery. having smaller boobs doesn't mean there is anything wrong with you or your transition. it's just what mother nature has given you. (also you're only 7-8 months in you may very well get larger in time.)
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soft tape measure link:
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Bra size calculator:
https://www.barenecessities.com/feature.aspx?pagename=fit_sizing
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Breast size database (nsfw)