Tyre pressure is measures in psi, bar tor etc. but for this topic let us take one which is most common in pakistan i.e the american used avoirdupois psi . Pounds force per square inch
Hence it is measure of force in relation to applied area i.e. per square inch. So 20 psi (for example) put force of 20 per square inch on the surface. Therefore it is slightly incorrect to say it is not in relation to volume, since it does have relation with area
Now that we have the context: Let us apply this to your query.
Typre pressure is inversely related to tyre size. So generally speaking a thinner typre e.g. cycle will have more psi , maybe 45-60 psi and broader the tyre the less pounds-force per Sq. inch it will exert.
Since you have upgraded your typre by 20mm (earlier was 145 now you say you have 165) and you say the manufacturer spec was 28 psi, technically speaking you should bring the pressure now to 27 psi. HOWEVER this is just 12" 13" rims in these it doesn't matter much PLUS you have kept the tyre profile (height) also same.
a bit lengthy explanation, but tomorrow you may upgrade to a bigger car - will come in handy
If you really care for your car , buy and keep a air pressure guage in house and check the pressure before leaving home or after a car's been parked for about 1-2 hours. Tyre pressure is always checked when tyre is cold. After checking you make the pit stop at the tyre shop and inflate the air to required amount. Alternatively make that pit stop at the first tyre shop you find.
P.s. You have eco tyres - so i hope you intentionally went for economy, since eco / roll easy tyres use silica in the tyre compound which reduced rolling friction but also impacts road grip! - hope you are aware.
The average self deflation in lahore, Karachi and Islamabad is about 4 psi per month (assuming rims are good quality and tyre walls are strong no damage or wear on walls).
Happy rolling.