Why Bigson and not Bigton?
Because the ת in בִּגְתָן doesn't get a דגש.
However the only reason ת in וָתֶרֶשׁ has no דגש is because it is preceded by a ו, just like the ב in וּבִנְיָמִֽן loses its דגש because of the ו which comes in front it the name Binyomin. בגדכפת in the beginning syllable or a word (as long as the preceding word doesn't end with an open syllable which connects to the word starting with בגדכפת) will get a דגש.