BCl₃ does not exist as a dimer but BH₃ exists as B₂H₆ because

Boron trichloride 3 BCl does not exist as a dimer but boron trihydride 3 3 B H exists as 2 6 B H because the large size of chlorine atom does not fit between small- sized boron atoms, while small-sized hydrogen atoms occupy the space between boron atoms. And in the solution state boron in boron trihydride accepts lone pair of electrons from oxygen and nitrogen to get the stability.