The safety profile of Ivermectin is ridiculously safe. A therapeutic dosages is 0.2mg/kg in humans. The estimated LD50 is >15mg/kg (actual ld50 is unknown).

To put this in perspective, a 250lb human would have to consume 15 tubes of the horse paste in one sitting to get a 50% chance of dying... Neurotoxic symptoms probably start around 2mg/kg. This would be 2 tubes of horse paste for 250lb human.

Assuming your vet is correct, I could see how this would be a problem for using a higher dosage in smaller animals. According to https://parasitipedia.net/index.php?option=com_content&view=article&id=2344&Itemid=2996 it is recommended to not use the horse paste in cats for dosages above 0.5mg/kg.