mjhjmtu

Consider the following MWE:

documentclassscrartcl

usepackagemathtools 
usepackage[upright]fourier 

begindocument

begingather*
 sigma_i+sigma_ngeqslanttauleft[nu_sigma^Q(i)+nu(R_i^-1cupn)-sum_kin R_i^-1sigma_kright]+(1-tau)left[nu_sigma^Q(n)+nu(R_n^-1cupi)-sum_kin R_n^-1sigma_kright]
endgather*

begingather*
 sigma_i+sigma_ngeqslanttauBigg[nu_sigma^Q(i)+nu(R_i^-1cupn)-sum_kin R_i^-1sigma_kBigg]+(1-tau)left[nu_sigma^Q(n)+nu(R_n^-1cupi)-sum_kin R_n^-1sigma_kright]
endgather*

enddocument

As you can see in the screenshot below, the squared parentheses of the two parts of the expression are not of the same size. I've been playing around to get them of the same size, but it turns out that I can only do so by commenting usepackagefourier, something I would personally like to avoid. Using left[right] is too big (case 1), whereas using Bigg[Bigg] (the biggest I know) is not big enough (case 2).

My question is thus simple: how can I get them of the same size without dropping usepackagefourier?

Thank you all very much for your time and effort.

What is causing you this trouble is the i lowered a tad too much:

There are a few approaches to overcome this and get the delimiter size you want. To name a few:

• 
  

  You can smash the i to make it have zero depth and height, then it won't be a problem anymore:

  
  
  

  beginequation*
   sigma_i+sigma_ngeqslant
   tau left[nu_sigma^Q(i)+nu(R_i^-1cupn)-sum_kin R_smashi^-1sigma_kright]
   +(1-tau)left[nu_sigma^Q(n)+nu(R_n^-1cupi)-sum_kin R_n^-1sigma_kright]
  endequation*
  

  
  


• 
  

  You can also change TeX's delimiterfactor and delimitershortfall and let TeX adjust the delimiters accordingly:

  
  
  

  beginequation*
   delimitershortfall=7pt % Found by trial-and-error
   delimiterfactor=810 % Move outside the `equation*` to make the effect global
   sigma_i+sigma_ngeqslant
   tau left[nu_sigma^Q(i)+nu(R_i^-1cupn)-sum_kin R_i^-1sigma_kright]
   +(1-tau)left[nu_sigma^Q(n)+nu(R_n^-1cupi)-sum_kin R_n^-1sigma_kright]
  endequation*
  

  
  


• 
  

  Or you can define a new size with amsmath's (loaded by mathtools) bBigg@. Note that simply using BIG you get a different spacing around the delimiters.

  
  
  

  makeatletter
  defBIGbBigg@3 % big = 1; Big = 1.5; bigg = 2; and Bigg = 2.5
  defBIGlmathopenBIG
  defBIGrmathcloseBIG
  makeatother
  
  beginequation*
   sigma_i+sigma_ngeqslant
   tau BIGl[nu_sigma^Q(i)+nu(R_i^-1cupn)-sum_kin R_i^-1sigma_kBIGr]
   +(1-tau)BIGl[nu_sigma^Q(n)+nu(R_n^-1cupi)-sum_kin R_n^-1sigma_kBIGr]
  endequation*
  

  
  

Full code:

documentclassscrartcl

usepackagemathtools
usepackage[upright]fourier

begindocument

beginequation*
 sigma_i+sigma_ngeqslant
 tau left[nu_sigma^Q(i)+nu(R_i^-1cupn)-sum_kin R_i^-1sigma_kright]
 +(1-tau)left[nu_sigma^Q(n)+nu(R_n^-1cupi)-sum_kin R_n^-1sigma_kright]
endequation*

beginequation*
 sigma_i+sigma_ngeqslant
 tau left[nu_sigma^Q(i)+nu(R_i^-1cupn)-sum_kin R_smashi^-1sigma_kright]
 +(1-tau)left[nu_sigma^Q(n)+nu(R_n^-1cupi)-sum_kin R_n^-1sigma_kright]
endequation*

beginequation*
 delimitershortfall=7pt
 delimiterfactor=810
 sigma_i+sigma_ngeqslant
 tau left[nu_sigma^Q(i)+nu(R_i^-1cupn)-sum_kin R_i^-1sigma_kright]
 +(1-tau)left[nu_sigma^Q(n)+nu(R_n^-1cupi)-sum_kin R_n^-1sigma_kright]
endequation*

makeatletter
defBIGbBigg@3
defBIGlmathopenBIG
defBIGrmathcloseBIG
makeatother

beginequation*
 sigma_i+sigma_ngeqslant
 tau BIGl[nu_sigma^Q(i)+nu(R_i^-1cupn)-sum_kin R_i^-1sigma_kBIGr]
 +(1-tau)BIGl[nu_sigma^Q(n)+nu(R_n^-1cupi)-sum_kin R_n^-1sigma_kBIGr]
endequation*

enddocument