thanks to strenuous workout and determination, I have lost around 15 kilos.
Should I go to the embassy and ask for a new visa now?
You certainly can, but it is probably not necessary. Immigration officers know that people gain and lose weight. Plus, the US probably has your fingerprints on file, so they won't have too much trouble matching you to your earlier entry on the same visa. The worst that is likely to happen is having to discuss your weight loss with the immigration officer, and unless you're very self conscious about it, that's not particularly bad.
At Photo Frequently Asked Questions, the US State Department says:
New photos are only required if your appearance has significantly changed from what is in your photo. Growing a beard or coloring your hair would not constitute a significant change. If you can still be identified from the photo in your current passport or visa application, you do not need to apply for a new passport or submit a new photo for your visa application. However, you may have to apply for a new passport or submit a new photo for your visa application if you have:
- Undergone significant facial surgery or trauma
- Added or removed numerous/large facial piercings or tattoos
- Undergone a significant amount of weight loss or gain
- Obtained a new gender identity
The acceptance of your photo is at the discretion of the U.S. passport agency where you apply for a passport or U.S. embassy or consulate where you apply for a visa.
This is only about whether you need a new photo for a new application, not whether you need to get a new passport or visa prematurely because of a change in appearance.
It does suggest that you may want to consider whether your weight loss is "significant." My guess is that it isn't likely to be if the amount is 15 kg. Perhaps you should show your photograph to some people and ask whether they recognize it as a photograph of you. Or, if the $160 fee isn't too much for your budget, and you want to avoid worry, you can just apply for a new visa anyway.
But border officers see people all the time with passports that may be as old as ten years, and manage to identify them despite weight loss, weight gain, and changing hair styles, among other things. I don't suppose that a 15 kg weight loss is that far out of the ordinary for them.