Is the reward of fasting commensurate with how difficult it is?
Question

Is the reward for fasting all the same with Allaah, or is it commensurate with the degree of difficulty encountered by the fasting person? For there are those who fast in cold lands who do not feel thirsty, whereas others fast in hot lands. This is apart from the good deeds that accompany the fast; rather I am referring to fasting only.
Praise be to Allah.If difficulty is an inherent part of the act of worship, and it cannot be done without encountering this hardship, then the greater the hardship, the greater the reward. Hence the Prophet (peace and blessings of Allaah be upon him) said to ‘Aa’ishah (may Allaah be pleased with her): “You will have a reward commensurate with your hardship and spending.” Narrated by al-Haakim and classed as saheeh by al-Albaani in Saheeh al-Targheeb wa’l-Tarheeb, 1116; a similar report also appears in al-Saheehayn. 
Al-Nawawi said in Sharh Muslim: 
The words of the Prophet (peace and blessings of Allaah be upon him), “commensurate with your hardship and spending” – the apparent meaning is that the reward is increased if more hardship and spending are involved. What is meant is hardship which is not criticized in sharee’ah (i.e., reasonable effort), and the same applies to spending. End quote. 
This principle – that the reward is commensurate with the hardship – does not apply in every case. Rather there are some deeds which are easy but the reward is great. 
Al-Zarkashi said in al-Manthoor fi’l-Qawaa’id (2/415-419): 
A good deed that is done more and involves more hardship may be better than one that is not like that. In the hadeeth of ‘Aa’ishah (may Allaah be pleased with her) it says: “Your reward will be commensurate with your hardship.” But a small deed may be better than a great deed in some cases: 
·Shortening the prayer is better than performing it in full for a traveller
·Praying once in congregation is better than offering a prayer on one’s own twenty-five times
·Making the two rak’ahs of Fajr short is better than making them long
·Giving the sacrificial meat in charity after eating a few mouthfuls of it is better than giving all of it in charity
·Reciting a short soorah in prayer is better than reciting some soorahs, even if they are long, because the Prophet (peace and blessings of Allaah be upon him) usually did that.
End quote.