Class DC.BigJSON Extends %RegisteredObject
{

ClassMethod Test(filename)
{
	if ..SaveToFile(..MakeJSON(), filename) {
		write "Save OK",!
		write "Size ",##class(%File).GetFileSize(filename),!
		
		set input=##class(%File).%New(filename)
		set sts=input.Open("RS")
		if sts {
			set json={}.%FromJSON(input)
			set iter=json.%GetIterator()
			while iter.%GetNext(.key, .val) {
				write "key=",key," size=",$l(val)," data=",$e(val,1,10)_"...",!
			}
		} else  { write $system.Status.GetOneErrorText(sts),! }
	}
}

ClassMethod MakeJSON()
{
	set obj={}
	set obj.text1=$tr($j("",3600000)," ","a")
	set obj.text2=$tr($j("",3600000)," ","b")
	set obj.text3=$tr($j("",3600000)," ","c")
	quit obj
}

ClassMethod SaveToFile(obj, filename)
{
	set file=##class(%File).%New(filename)
	set sts=file.Open("wnu")
	if sts {
		do obj.%ToJSON(file)
		do file.Rewind()
		use file.Name 
		do file.OutputToDevice()
		do file.Close()
		quit 1
	} else { quit sts }
}

}

The size shouldn't be a problem

USER>do ##class(DC.BigJSON).Test("/tmp/test1.txt")
Save OK
Size 10800034
key=text1 size=3600000 data=aaaaaaaaaa...
key=text2 size=3600000 data=bbbbbbbbbb...
key=text3 size=3600000 data=cccccccccc...

The first part (Base 64 encoding is not able to encode ... unicode (2 byte) characters)  is correct. The second part (data-->utf8-->base64 and base64-->utf8-->data) is correct only if there is an agreement beetwen the sender and receiver about the double-encoding (utf8+base64).

If I was told, I get a base64 encoded file then I expect a file which is only base64 encoded and not a mix of several encodings including base64. A simple way to encode your document could be something like this:

ClassMethod Encode(infile, outfile)
{
    // file-binary reads bytes and not characters
    set str = ##class(%Stream.FileBinary).%New()
    set str.Filename = infile
    set len = 24000 // len #3 must be 0 !
    set nonl = 1    // no-newline: do not insert CR+LF
    do str.Rewind()

    open outfile:"nwu":0
    if $test {
        use outfile
        while 'str.AtEnd { write $system.Encryption.Base64Encode(str.Read(len),nonl) }
        close outfile
    }
    quit $test
}