My business net income varies greatly year to year, but I have had a couple of years like that, and 2018 will be even more so I think.
It sometimes feels strange having to pay the IRS more money in taxes than I spend on myself each year, and seems to somewhat make me question how frugal I should be on myself.
It does feel strange given I'm not used to it either, as just 5-6 years ago I had a tax year where I owed pretty much zero except for payroll tax, but I'm not sure why you would question your frugalness as it doesn't change the reasons at all. Making so much money is a blessing that just allows you to get to FIRE so much faster (allows for a higher savings rate at the same or even higher expenses), and the high taxes (relative to what you spend) is just a corresponding drag on that faster pace.
It does make tax planning much more important given the effect it can have. For 2017 I made about 7.5x my annual expenses. Basically of what I made for 2017 I spent one years expenses, saved 4.5x expenses, and paid taxes equal to 2x expenses. For a moment I selfishly get pissed I'm handing over what my family could live on for 2 years, but that only lasts a moment as I also enjoyed the other end of things of paying so little tax when things were tight and I know how lucky I am.
I also pour it into index funds. I max out the solo 401k ($54k last year) which is of great help with taxes. For last year and this year I will be making large contributions of appreciated stock to a Donor Advised fund. Basically the plan will be to use this fund throughout decades of early retirement to make my charitable contributions from, as those will be worth little to nothing in tax deductions to me then but are of extremely great value now, to the level of these contributions only truly costing me half of what I put in after tax. Without these two moves my taxes would have cost another half-year of expenses, so the effect can be great.
If I was to do this for a couple more years I would have looked into a defined benefit pension plan to lower the tax bill even more, but I think my business winding down after this year and the extremely fluctuating net income I face made this difficult to take on without risking penalties.
Now if AFTER you've reached your comfortable fire number AND you're still making a lot, then I think its time to reevaluate your frugalness and/or your continued work, but before that if your happy with what you spend now I don't know why you'd change whats working. I think some see a high income and savings rate and have the intent to continue working and making the same money to the point they extrapolate that they are over-saving...but a lot of things out of your control can happen, so I've continued with the theory you get to FIRE first and then if you find yourself over-saving at that point you spend/give more and do so with peace of mind