Many people don't understand how payrolls are computed.
The important thing to note is that, at the end of the year, the tax, EI, and CPP that you owe is based on the total you make, and it all works out in the end. If you do a lot of overtime, and one pay period shows large deductions, it gets balanced out when you do your taxes, and get a refund.
There is, for example, a maximum CPP premium payable, once you reach the limit, your employer (if they use proper payroll procedures) is supposed to stop deducting it, or you get it refunded anyway.
Taxes and EI premiums are deducted each pay period based on the assumption that each pay period is typical, so for example, if you work a lot of OT one period, the taxes deducted will be high, because the taxes are deducted based on the idea that you make this income every pay period, but at the end of the year, you only pay the taxes based on your actual income, and you get the rest back.