We have a show type Labrador too - and she would be the size of a house, if she had her way! As a previous poster said - it is our job to control her diet and exercise to make sure she stays a healthy weight - and it is perfectly possible, though you do have to be careful about leaving food within reach, as it will get stolen in a heartbeat! The list of things our dog has stolen and eaten ranges from steak and salmon, through several tubs of butter, to ginger biscuits and raspberry turnover and even the end of a bag of flour - which was hilarious - flour plus labrador drool turns into glue, and she made some very funny faces whilst trying to de-gunge her mouth!
It didn't help when we got a second dog - a Labrador pointer cross - who is very bright and can open doors - that's how they got the gingerbread biscuits - I had shut the kitchen doors firmly, whilst I nipped to the Coop for icing sugar, but ddog2 broke in so they could both feast!!
Ddog1, the Lab, did once get into the cupboard where her sack of dry dog food was kept, and ate 4.5 kilos of it, before we noticed - and we only spotted what was going on because she was so full she'd stopped eating to be sick (and make space for more). She was a brown barrel on legs - and we didn't feed her for days!
She was fine, though - she has an utterly bomb proof digestion - she has stolen and eaten things that are supposed to be toxic for dogs (chocolate, mince pies), and not turned a hair.
This all makes us sound like very careless owners - I promise we aren't - Labradors are very clever, when it comes to obtaining food - even brown Labs, who are not the brightest animals in the world.