The problem with useing a battery that is too small is that if the battery is not able to supply the motor with the power it needs is that the voltage will likely sagg and the LVC circuit in the ESC will kick in and the motor will stop. This will likely happen when you need the power to get out of trouble and you could loose your plane also, is it worth it? Also if you try to overload a Lipo it will soon heat up and puff and be ruined. Best get the right battery for the job!!!!!!!

Happy Flying
Bob

One possible way to use your small battery might be to prop down the motor so it wont draw as many amps to harm the battery. You will need a wattmeter for this and also know how many amps the battery can supply and then start from there.......