For word bearer legion, Butcher's nails states that around 40 to 50k word bearers are with the world eaters. In KNF i think it was around 100k that where sent to Calth but can't remember for sure. So that would put them at between 150k and 200k.
Also in KNF while it is stated that UM are at 250k, if I remember correctly i think estimated loss where at 130k, leaving the UM with still a good force of around 120k. It's quite possible that since the events of Calth UM legion size dropped and that another legion would have more troops, namely DA.
Even if the Word Bearers where succesful in crippling the UM, it's true that Horus would not want to meet the DA in a front war if the estimates of Brother-Sergeant are correct.