The reason the dinosaurs were so big was mainly due to the higher levels of oxygen available in the atmosphere. According to this, this was also the reason why all arthropods were also much bigger than today. This because they "absorb" oxygen through their carapace. This method of breathing is limited and thus the higher oxygen level is what enabled them to achieve the size they did.
This also applies to all other animals that breathe or need oxygen. A higher oxygen level while all other parameters are maintained, enable an increase in size.
In your case, the only real thing your planet would need is not an increase in size but an increase in oxygen levels. This can easily be accomplished with greater flora or more "efficient" plant life.
I read an article that I cannot find now that stated that one of the factors for greater oxygen levels in the prehistoric era of the dinosaurs and even before them, was in part due to the fibers and cells in trees of that time. Bacteria and smaller insects didn't know how to process these substances and later evolved to do so. But when they could not, the dead trees simply lay there and didn't decay as they should. This lead to less carbon dioxide emissions as well, also being a cause for bigger arthropods and the like.
To be able to produce a planet that could sustain massive animals such as monster hunter animals, increase oxygen levels.
Their size, as mentioned being ~400meters, should be irrelevant since a planet the size of earth is still absolutely massive. If you want to, just increase the size to your liking and change the composition of the planet to have a mass that equates to 1G, but is bigger in volume. In short, greater volume, lower density.