My take is:
In the beginning, I rise; in the end, I fall.
In the beginning, I am large; in the end, I am small.
In case of balloons filled with helium they initially rise but eventually they fall after loosing the gas. When we inflate a balloon its large and after it looses the gas it becomes small.
I firstly expanded, but I will soon contract.
I was lucky to not make a bang and lie cracked.
When we fill the balloon with air or helium its rubber expand and after it looses the air it contracts. In this case the balloon was lucky that it wasn't filled more than its limit which could have led in bursting of balloon which may have lied in corner there after.
In the middle, I float; in the middle, I drift.
In the middle, my string and I experience lift.
In the middle of the air it floats and if wind blows up it drifts. In the air the string attached to the neck of the balloon filled with helium it experience a lift as helium filled balloons rise in air.
The middle is the event we currently undergo,
And there is no need to fear that I might outgrow.
Flying in air is the event it is currently undergoing. And till the time its in the limit of the atmosphere it wont deflate(unless it bursts due to some reason)
Unfortunately, the middle is finite.
Can you guess what I am before I fall from my height?
But soon it fill cross the atmospheric limits and will burst due to pressure. Balloon it is as it falls after bursting from the sky.
The balloon is in mid air and is getting a temporary view from the height. And it can burst or popped with some sharp object or from a blow from show with some sharp edge.