What defines the bit width of an instruction set isn't connected to data bus width, as different implementations of the same instruction can have different data bus widths.
That's news to me. When I doing electronics as a teenager in the 1980's, an 8-bit processor had eight data lines, a 16-bit processor had 16 data lines, and a 32-bit processor had 32 data lines. I recently saw a 64-bit microcontroller that implemented one-half of the data bus (32 bits) as four 8-bit serial ports (four pins). I'm not sure if that's a four-bit or two-bit design.
Again, there's the width of the processor's external bus, the width of the processor's internal signal paths, and the width of the registers and instructions of the instruction set the processor implements. Nothing ties the first two of those to the third of those, as evidenced by various models of the System/360 series (the I/O interface had 8 "bus in" lines, 8 "bus out" lines, and various control lines; the processors had internal signal paths ranging from 8 to 32 bits for integer and address operations; the instruction set had 32-bit general-purpose registers and arithmetic instructions and 24-bit physical addresses), the Motorola 68000 series (the 68000 and 68010 had a 24-bit address bus and a 16-bit data bus, and a 16-bit ALU for data operations; the instruction set had 32-bit registers and arithmetic instructions and 24-bit physical addresses, extended to 31-bit physical addresses with the 68012 and 32-bit physical addresses with the 68020 and subsequent processors, which had 32-bit internal data paths), and the 8086/8088 and 80186/80188 (same processor core in the 86 and 88 variants, just a different external bus; the instruction set had 16-bit registers and arithmetic instructions).
It's about more than just the electronics; it's about the software, and that mainly involves the instruction set, with the external and internal data widths being a performance issue rather than a pure functionality issue.
So, from the 1960's (and maybe earlier) to the present day, you could, for example, have a processor with an 8-bit data bus and 16-bit, 32-bit, or 64-bit registers and arithmetic instructions and 16-bit, 24-bit, 32-bit, and 64-bit physical/virtual addresses.
The 8088 was a processor with an 8-bit data bus and everything else 16-bit; as Intel's manual says, "The 8086 and 8088 are closely related third- generation microprocessors. The 8088 is designed with an 8-bit external data path to memory and I/O, while the 8086 can transfer 16 bits at a time. In almost every other respect the processors are identical; software written for one CPU will execute on the other without alteration." They also note that "The high performance of the 8086 and 8088 is realized by combining a 16-bit internal data path with a pipelined architecture that allows instructions to be prefetched during spare bus cycles.", so both are internally 16-bit implementations of the 16-bit instruction set.