At equal aptitude, employees with a higher engagement with the company are more valuable.
They have finite resources, and obviously those resources will be allocated in a way that benefits the company the most.
So yeah, you should expect your coworkers to get a bigger raise, and you should be fine with it. They won't love you all equally, it's a company.
In your assessment of the situation, the equation is:
value brought to company without overtime + value brought to company by doing overtime = salary
value brought to company without overtime + value brought to company by working overtime = salary + overtime pay
having employees that are available for overtime provide burstable capacity at a cost that scales linearly.
in comparison, adding extra employees due to a temporary requirement is a worst case scenario. hiring is very expensive, takes time, you need extra infrastructure, training, you may operate unnecesarily at excess capacity and there are diminishing returns.