NEAT: A huge factor in fat loss and 'boosting your metabolism'.
NEAT (Non-Exercise Activity Thermogenesis) is simply calories burned by activity that isn't specifically "exercise". Walking, labouring, housework, fidgeting, playing with kids or pets, literally any movement.
Metabolism is just how many calories your body expends per day, including digestion, breathing, thermoregulation (temperature control), LIVING. A person in a coma still burns calories!
We burn surprisingly few calories in a typical gym or cardio session. However, NEAT can vary significantly between individuals, with some burning close to 2000 additional calories daily without even realising it (due to their occupation and lifestyle). Thanks @bdccarpenter for the cool infographic.
That friend you have who seems to eat like a horse and never gain weight? It's highly likely their NEAT is significant. Especially if combined with regular exercise, then the result is that coveted "fast metabolism".
(Note: it's also common that they don't eat as much as they seem to)
Having more body mass does increase metabolism (ie a bigger frame, more muscle and/or more fat) for the same amount of movement (or lack of).
However, more muscle doesn't burn as many calories as we'd like. One extra kg of muscle mass burns approximately 13 cals per day at rest.. That bigger frame is most helpful if it moves!
Practical recommendations if your job has you driving a desk? Take the stairs at every opportunity; walk at every opportunity; set an hourly alarm to get up and move around; dedicate 10+ minutes to chasing your pets a few times a day. Use a pedometer to track daily steps and aim for 10000+
This photo was taken after we had just walked ~8km (much of it along a windy back beach) for fun on holiday. Sounds crazy to some... A fun adventure to others, and an enjoyable step towards eating more food!
Dietary control is still the #1 factor in successful fat loss. However, increasing energy expenditure can mean you get to eat more food and still be in a caloric deficit (which is the single defining factor in losing weight). Briefly continued in comments: