That is really a very small difference given that your income increased 60%. Going from $25K to $40K means a much larger percentage of you income is above the standard deduction amount and is taxable. I suggest you compare your tax returns side by side and look at the what changed in addition to your refund.
More income=higher tax=less refund or more tax due. Your withholding didn’t increase enough to cover the tax on your higher income.
Nothing is the same as last year. There are many differences. If you have kids they are a year older. That can make big changes. Also the tax laws and brackets might have changed. The withholding tables change each year. Each year is separate.
Why is my refund lower?