@thejessiekirk It's not always the case with injections: just the ones that trans masc folk in the UK tend to be given.
The 2 most-commonly-prescribed injections are:
Both are compounded at 250 mg/mL, but they're quite different otherwise in terms of chemical make-up, viscosity, and volume.
Sustanon is a 1 mL injection mix of refined arachis oil (peanut oil) and benzyl alcohol, containing:
- 30 mg testosterone propionate
- 60 mg testosterone phenylpropionate
- 60 mg testosterone isocaproate
- 100 mg Testosterone decanoate
It is typically administered every 2-4 weeks.
Nebido, conversely, is a 4 mL injection containing 1000 mg testosterone undecanoate, suspended in a mixture of refined castor oil and benzyl benzoate. It's typically administered every 10-12 weeks.
The main difference between these, other than the different esters of testosterone, is the viscosity of the carrier oil used.
Figures vary and there's no universal unit used for viscosity, but in all of these, castor oil is significantly more viscous than peanut oil, or MCT oil (which is commonly used in the DIY / homebrew community).
Don't know if this'll help or not, but we thought you might find it interesting 
