Highly exothermic is only adjective. It's meaning the energy releasing for environment is high. Temperature rising is occurring at exothermic reactions. Using Hess rule, we can find out the whether reaction is exothermic or not. If it is exothermic, the value of enthalpy take negative sign. If it is endothermic, it reserves a positive sign for enthalpy. And enthalpy is also an extensive property. The amount of energy releasing and absorbance depend on amount. Therefore, high amount of reactant cause to highly exothermic reaction. @Abdul Hakim Hatta

Abdul Hakim Hatta, "Highly exothermic reaction" is an ambiguous term. There is also no point in comparing the thermal effects of chemical reactions with nuclear decay, because this is a completely different scale. Moreover, in addition to the heat effect of a given reaction, the heat capacity of the system is important. Only these two factors allow us to assess the scale of the temperature increase in a given reactor system.

I agree with Poornima Nadun and Mirosław Grzesik .

The terms "highly, lowly, much, less" are relative in engineering. High, low, much less in comparason to what. Highly exothermic = is simply exothermic, you can just give the enthalpy value.

The heat of reaction alone is not sufficient to judge the temperature rise in the system, and this is important. It is necessary to know the heat capacity of the system and whether it is being cooled or not.