Am I wrong?
When I started 40 years ago I bought a lot of beeswarms. 4kg =8 lbs occupy 1,5 langstroth hives, but it can use whole 2 box and carry honey one box and lower one was full of brood. When brood started to emerge 4 weeks from start 2 box was full of bees soon. It depens how long is you summer but id you have 2 box hive at the beginning of May and it has 1,5 box brood, at the end of June you will have 4-5 box bees and you have a good honey collector.
If you start with 3 lbs (1,5 kg) difficult to say how fast it developes but not fast. Perhasp it is able to collect honey in August.
Small hive is difficult because if it get well honey hive will be soon full brood and honey, and queen must drop down it's laying. In bigger hive you are able to extract honey and give more space.
Beekeeping is expencive hobby. The extra price you pay for 6 lbs is nothing in the whole. And in later summer you will get one hive more from your first if you like. An its is good to be more hives than one.