Sanjay sir...
I too have a similar experience...  But I feel low Kv motor with large props actually consume more current... 
The increase in flying time I feel can be attributed to reduced throttle...  We tend to fly at lower throttle getting enough thrust because of larger props...
Please correct me if am wrong

Which electronics would you suggest for flying a 1metrr length wingspan trainer plane and body length of 30 inch

Hello Sir,

how to chose prop and motor combo, is there any theory or it is all abut experience.

Above post merged into existing topic. Please do some searching and reading here... as this topic is covered in detail in this and other threads.  If you have specific doubts even after reading, please feel free to ask.

If every newcomer asks the same questions already answered, it would lead to a lot of duplication of content, as you can imagine.  But any clarification on existing content would add value to the forum discussions.

Welcome to RC India