Per this excellent answer on Memory Alpha;
In the 2364 Starfleet Academy entrance exam, one of the questions
dealt with intermix ratios:
"The matter and antimatter tanks on a Galaxy-class starship are
nine-tenths depleted. Calculate the intermix ratio necessary to reach
a starbase 100 light years away at warp factor 8."
This was a trick question as there was only one ratio with
matter/antimatter: 1:1. (TNG: "Coming of Age")
In the same year, after the Shuttlecraft 13 crashed on Vagra II, the
USS Enterprise-D was rushed to have warp drive operational. In the
start-up sequence, prior to powering up the engine core and injecting
reactants, the matter-antimatter injectors were primed to a ratio of
25:1. Minimal warp speeds were recommended, however Captain Jean-Luc
Picard ordered warp 8. (TNG: "Skin of Evil")
Jadzia Dax ordered the intermix be manually set to 0.01, prior to the
warp chamber initialization, on the USS Defiant after it had been
damaged by a cascade virus. (DS9: "For the Uniform")
Several aspects of the intermix ratio are further explained in the
Star Trek: The Next Generation Technical Manual (pages 61-62).
According to the text, the 1:1 ratio applies when traveling at warp 8
or faster. When traveling at lower speeds the intermix ratio is
different - a ratio of 25:1 is only used when the core is online in an
idle state, prior to full startup. At warp 1 it is tuned to 10:1,
gradually changing until it reaches 1:1 at warp 8.
This description tallies nicely with the real-world effects of adding extra matter to an antimatter/matter reaction since the extra matter would become hotter and expand as energy is input.
If the reaction chamber needs to be at a specific pressure before high levels of energy can be created, then adding extra matter (e.g. to "prime" the chamber) would be an excellent way to achieve this.