Usually exercise is required to lose weight and a byproduct of that is you improve fitness but that came from the work not the weight.
I agree to a point carrying the weight has an effect in terms of it needing to be 'fed' or 'sustained' by the body during a workload but losing 5lb is not going to help much unless you are right on the edge of peak physical condition which not many will be.
You don't need to exercise to lose weight, so I still ponder the original question, at smaller levels would you notice it more on the bike or yourself.
Whilst it's true that you don't necessarily need to exercise to lose weight, think about the demographic - most active people, you know the sort ride bicycles, tend to see some improvement in fitness and conditioning having lost a reasonable amount of weight - because there tends to be something of a feedback loop going on.
Point being, you can never ignore the psychology and behaviourism going on.
In my experience, weight savings on bikes tends to be a progressive thing - perhaps it's different if you've spent a lot of money all at once making your mtb more fragile - but that's probably atypical - and truth be told, 30 minutes in, I suspect you're more likely to realise the benefit of weight loss in yourself, and most likely, some uncoincidental improvement in fitness, than a pound or two off your bike.
And if it's more than a couple of pounds off the bike, you have to question it being an ideal candidate, or whether you've just made your off-road bike a little less fit for purpose.