If your question is what is actually linearly depending on the concentration, the band area of an absorbance band or the peak value, then the answer is: It is strictly speaking the band area, but often also the peak value can be used if the transition moment is not too strong. If you are interested why, then have a look at Article Beer's Law‐Why Integrated Absorbance Depends Linearly on Concentration

If this is not the answer you expected, maybe you can explain in a little more detail what you mean by "big and small figures"? What are you comparing, before and after a reaction? Certainly, if a reaction takes place, a band related to a reactant should decrease in intensity and area or vanish completely.

Thomas Mayerhöfer yes, what i mean is before and after a reaction. So, based on your answer, the ratio number should be decrease if a reaction takes place right?

Thank you so much for recommended article.

My pleasure! You certainly can also use the ratio to observe the reaction progress if you use one band of the reactant and one band of the product.

By using peak area of Vibrational banding of bonds in FTIR, we can get information about relatively concentration of cations or shifting of ions from one void to another void during applying physical condition like temperature, pressure etc.

For Example- Consider A spinel structure nanoparticles Mn3O4 having two vibration peaks at tetrahedral sites and octahedral sites, if we increase the annealing temperature, shifting of Mn2+ or Mn3+ will done, so by peak area ration (octahedral to tetrahedral or vice versa), we can detect the shifting the ions from one site or void to another site or void.