By default, msdb is not really using the log as it's set to simple recovery mode (truncates on checkpoint) so growth should be relatively small if at all. TempDb is used for a lot of different things so the file size should be increased. This has nothing at all to do with memory but everything to do with disk space.
Edit: and let's be clear, if you do increase the physical size of temp, it's permanent, not temporary.
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