Does a bigger load of ejaculation lead to a better chance of getting my wife pregnant?
The only exception is if your doctor has conducted a semen analysis and determined that you have oligospermia, in which case your doctor might recommend having sexual intercourse with your wife 3-4 times a week instead of every day.
The bottom line: Don't start any unproven 'methods' for increasing the chances of pregnancy without talking with your wife about your idea, and asking your doctor(s) if the method has been scientifically proven to work.
Here's what some experts have to say on the topic:
"...prolonged abstinence has little positive effect on conception and can actually decrease a couple’s chances of [getting pregnant] — even for men with low sperm counts. How so? While holding back does increase the number of sperm, ... abstinence longer than one day decreases sperm [motility]..." - Boosting Male Fertility (WhatToExpect.com)
Does frequent masturbation affect male fertility? by Erik P. Castle, M.D. - "Ultimately, having sexual intercourse ... several times a week will maximize your chances of getting your partner pregnant, whether you masturbate or not."
Also on the Mayo Clinic site: How To Increase Chances of Pregnancy: 5 Tips To Improve Sperm Quality
From PubMed Health - Infertility: Overview
How To Increase Chances Of Pregnancy: 5 Tips To Improve Sperm Quality - an interview with Mayo Clinic doctors by Parent Herald online magazine.
Your actual sperm count will matter as well as volume to some degree, as well as her level of fertility. If your wife is not currently releasing a fertile egg nothing else at all matters of course. If there no fertile egg there to be ready, then volume & count mean nothing. You have to account then for timing as well of course. But you can have a high sperm count with low volume & have better chance than high volume with a low count.