Using Photoshop CC, I am importing an otherwise finished H.264 960 x 540 video as a smart object and adding one tiny image for a few seconds and a transition. When I render the video, the file is over twice as large as the original (99KB to 224.6 KB). The only difference I can see is that audio saved in the original as 44.1 kHz mono, is saved in the new version as 48 kHz stereo. Please can you help me find out how can I render the video whilst maintaining the original quality without increasing the file size so much? Thank you.
You will have to read up on video compression on a general level. This is not something anyone can answer in a short forum post. It all comes down to what encoding settings you actually used when rendering, none of which you have tolds us. also be aware the recompressing an already compressed clip will always result in more compression artifacts and loss of quality with the same output settings. Inevitably you will actually want to use higher datarates to not completely degenerate your existing footage. Well, anyway, seriously do some web search. Plenty of stuff to find on the subject.
It's unlikely to be the audio. It's down to the render settings you use. H.264 is a good option. It is also handy to use presets like YouTube, which will select sensible settings. You should also think about running the rendered video file through Handbrake, which works wonders reducing file size with minimal loss of quality.