The size of needle is fine for sewing on denim so unless the needle is bent (and even so) I doubt that is the problem. Breaking upper thread typically has more to do with the threading, ie wrapped around some place along the way or misthreaded somehow, of even if you somehow got knots or that it has somehow got thread around the spindle for the spool. So first recheck the threading. Best to do by simply totally unthreading it, looking closely at the spool and winding and then rethreading it again. Make sure your upper tension hasn't been accidentally tightened too.
Next check the bobbin and bobbin thread. First pull on the thread. Is it pulling real tightly or does it flow out readily? Take the bobbin out and look at it. Check the tension also. Is it too tight? That too can cause the upper thread to break.
Is the needle penitrating the material smoothly? Again examine the needle closely to see if it's developed a burr. Better yet just swap the needle with a new needle and try it.
The majority of the time it is one of these things that will cause the top thread to break.
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