I know that a significant weight of the human body (in Kgs) is contributed by microbiota. I also know that antibiotics can often be broad spectrum and kill all the bacterias regardless of them being good or bad for us. So why doesn't antibiotic courses reduce our weight as the microbial population must've been demolished?
First, you present as fact that antibiotics therapy (say AT) does not reduce weight. To show this fact, a study must weigh persons before AT and after. Confounders to this measurement are 1. weight differences because of bladder content (up to around 1 liter urine = 1kg), 2. weight differences because of bowel content (100g-200g), 3. Food weight (up to 2kg and more). High salt foods can cause water retention that can easily reduce your urine output one day by 500-1000ml (=g), increasing it with the same amount the other day. So your confounders can add up to +/- 4kg easily. But what you want to measure is a change of 1kg.
This means that you need to account for all these. I don't know any study that did this. Please cite such a study before presenting as fact that there is no weight loss from AT.
Assuming there is no weight loss, there are several hypotheses that can be advanced: from the fact that no antibiotic (even broadband) can kill all bacteria, the remaining bacteria in the gut may simply feed off the highly nutritional content of those bacteria cells that were killed. Note that although we know which bacteria of those that we can cultivate in a lab are susceptible to AT, we don't know at all about those that we can't grow, and that is the majority of bacteria in the gut.
Antibiotics can cause weight loss, though, especially as they are used in the clinically, they are more likely to cause weight gain than weight loss.
This Lancet Infectious Diseases review is not a very good review in general, but it does catalogue many of the weight related changes in response to antibiotic therapy. You could use the references as a good place to start.
Re: @RStephen's answer:
This means that you need to account for all these. I don't know any study that did this.
Having run several phase I trials, I can tell you that we do account for all these factors. These trials are done to demonstrate safety and determine the pharmacokinetic properties of a drug (absorption, distribution, metabolism, and excretion). We keep volunteers in house, and ins and outs are carefully monitored. When we control intake, if weight loss does occur, it's water loss (through diarrhea) or water loss and decreased absorption (again, with diarrhea).
@RStephen was correct, though, that you're not going to get weight loss due to elimination of your microbiota. It is simply impossible to sterilize the gut. You can throw as much heavy artillery as you'd like at it, but the best you can do is change the composition.
Clinically, though most sick patients gain weight when you treat them, weight loss is not uncommon after broad spectrum antibiotic use (often clindamycin or metronidazole) typically with GI side effects (nausea, anorexia, diarrhea). When it's due to changes in the bowel flora, it can be overgrowth but not sterilization. You can read about that in this excellent JAMA review