Why do pediatric tracheostomies need up-sizing? i know kids of very diff sizes with same trach size.

Depends: Individuals have different diameter tracheae, different levels of scarring, even if they're similar in size. So a large child with lots of scarring will need a smaller trach tube than a smaller child with a large tracheal diameter.

Different airways: Different airways can grow differently. Additionally, there may be other factors involved such as scarring, granulation tissue, granulomas, that need to be taken into account. Airway size does not always depend on age.

Breathing Needs: As a child grows they increase their ventilation needs. A trach tube that is too small will limit ventilation and make it harder for the child to breathe. An adequate size trach tube must allow for ventilation during activity without resistance. If a child had to pull or retract to breathe the trach tube is too small.

Safety.: Pediatric trach tubes are small, by necessity. They are easily blocked by secretions and can result in total airway obstruction. So, as a child grows, larger tubes will fit into a larger trachea, and have less chance of total obstruction by secretions.

As baby grows: So does lung size and the need for more oxygen to fill those lungs and thus a larger conduit to bring more air in with every breath.

Optmum sizing is a : Neonatal and pediatric tracheostomy tubes have a very small inner diameter- can easily get blocked with secretions. As the child grows the smaller trach tubes can be limiting by creating an airleak around thus affecting an effective ventilation, can be easily dislodged risking a critical airway, affecting a child's growth. Optimum sizing of a tracheostomy tube is an art and a science!