A comma decimal separator that is bigger than a dot

You need an intelligent comma-package like icomma or ncccomma.

You can also adapt the code from Claudio Beccari's article in The PracTEX Journal, 2011, No. 1.

Add

\usepackage{icomma}

or

\usepackage{ncccomma}

to your preamble.

If you have none of those packages installed, you can add Claudio Beccari's code to your preamble (I have removed the comments and added \makeatother as the last line):

\makeatletter
\DeclareMathSymbol{\punctcomma}{\mathpunct}{letters}{"3B}
\DeclareMathSymbol{\decimalcomma}{\mathord}{letters}{"3B}
\AtBeginDocument{\mathcode‘\,="8000}
{\catcode‘\,=\active \gdef,{\futurelet\let@token\m@thcomma}}
\def\m@thcomma{\let\@tempB\punctcomma
\@tfor\@tempA:=0123456789\do{%
    \expandafter\ifx\@tempA\let@token\let\@tempB\decimalcomma
    \@break@tfor\fi}\@tempB}
\makeatother

Hopefully, this will help.

I don't know what intelligence is hidden in the mentioned intelligent packages, but I guess that it can be implemented by the four lines:

\def\mathcomma{\futurelet\next\mathcommaA}
\def\mathcommaA{\mathchar`, \expandafter\ifx\space\next\,\fi}
{\catcode`,=13 \global\let,=\mathcomma}
\mathcode`,="8000

without space after comma: $1,234$ with spaces: $1, 2, 3$.

\bye