Have you removed the inserts in your car seat? What are the limits of the car seat?
Our LO is 5 months and she’s been in the stroller attachment for a couple of weeks now x
It’s much safer to rear face until they are around 5 years old it’s the recommended age not the legal age but definitely the recommended age you can get rear facing car seats for up too 150cm height and 36kg weight, have a look on a Facebook group called car seat safety UK x
You can get a rear facing convertible car seat which are much bigger and don’t come out of the car. They turn into front facing and eventually booster seats. But your infant seat should be usable for a long time. Have you tried adjusting where the straps come out? They usually move up to accommodate more length. Take the pads and the backing off and have a look.
Which pram do you have? Most have the option to have the seat parent or world facing.