A preliminary comment: The 1 and 2 glyphs in \frac{1}{2} are rendered in \textstyle, while 123 in the denominator of the exponent term is rendered in \scriptscriptstyle. If the main text font size, or "normal size", is 10pt, then "scriptscript-size" is 5pt, i.e., it represents a 50% reduction in the (nominal) font size.
You may not have realized it, but what you are basically asking for is to discard one of the main distinguishing features of the Computer Modern (Roman) fonts: their optical scaling.
Optical font scaling is very different from linear font scaling. Their difference may not be readily apparent if the scaling factor is close to 1 (say, 0.9 or 1.1); however, it becomes very noticeable if the scaling factor is 0.5.
Linear, or ordinary, font scaling is what you appear to be looking for. Unfortunately, linearly scaled glyphs tend to look very "skinny" and become hard to read when their size is reduced by 50% -- which is what \tiny (in text mode) and \scriptscriptstyle (in math mode) perform.
Optically scaled glyphs, in contrast, apply the same scaling in the vertical axis, but take care to adjust the glyphs in the horizontal axis -- mainly by making the strokes relatively thicker, but also by increasing the distance (jargon term: the glyphs' "side-bearings") between glyphs in the horizontal axis. It would be a mistake to think that optical scaling merely involves a linear stretching along the horizontal axis.
In the following screenshot, the numerals on the left employ Computer Modern, and those on the right employ Helvetica -- which is a (sans serif) font that does not perform optical scaling by default.

The linearly scaled-down CM glyphs (in red) are spaced together much more tightly and have thinner strokes than their standard, i.e., optically scaled counterparts (in black). By construction, no such difference is present in the Helvetica numerals.
If you really, really cannot stand the optical scaling feature of Computer Modern, you're free to choose an alternative whose glyphs are linearly scaled. Just don't expect to get a high-quality typographic result if your document contains more than just one or two items in double-subscript or double-superscript positions.
\documentclass{article}
\usepackage{graphicx,xcolor,helvet}
\begin{document}
% "\scalebox" performs linear scaling
{\tiny 123}\kern-9.73pt\textcolor{red}{\scalebox{0.5}{123}}
\space
\sffamily
{\tiny 123}\kern-8.335pt\textcolor{red}{\scalebox{0.5}{123}}
\end{document}